Changed all LFSR_DATA/ATTR macros to create lvalues

I think what may be going on with the unexpected stack cost related to
struct passing, is something to do with scoping and how it interacts
with function inlining + shrink wrapping.

Compound-literals have a scope limited by the current statement, and
while temporary structs _should_ have a scope limited to the current
expressions, maybe this scope is getting messed up due to function
inlining?

Still smells like a compiler bug, but if this is true, wrapping the
struct-generating function calls with compound-literals should be more
robust at preventing unexpected stack increases in the future.

As a plus, this makes all LFSR_DATA/ATTR macros lvalues, which is nice
for consistency.

---

Unfortunately, it does seem like GCC 11 is not able to elide moving
compound-literals all that well. Repeatedly nesting trivial
compound-literals results in a measurable increase in code cost, even
though it should theoretically be a noop with optimizations.

This results in an unfortunate code size increase:

           code          stack
  before: 33780           2640
  after:  33868 (+0.3%)   2640 (+0.0%)

But at some point you have to give up trying to work around
insufficiencies in the compiler. I'll take 100 bytes of code over 100
bytes of stack any day.

lfs.c

@@ -1415,7 +1415,7 @@ static int lfsr_bd_progdata(lfs_t *lfs,
 #define LFSR_LEB128_DSIZE  5
 
 #define LFSR_DATA_LEB128_(_word, _buffer) \
-    lfsr_data_fromleb128(_word, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_fromleb128(_word, _buffer)})
 
 #define LFSR_DATA_LEB128(_word) \
     LFSR_DATA_LEB128_(_word, (uint8_t[LFSR_LEB128_DSIZE]){0})
@@ -1433,7 +1433,7 @@ static inline lfsr_data_t lfsr_data_fromleb128(uint32_t word,
 #define LFSR_LLEB128_DSIZE 4
 
 #define LFSR_DATA_LLEB128_(_word, _buffer) \
-    lfsr_data_fromlleb128(_word, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_fromlleb128(_word, _buffer)})
 
 #define LFSR_DATA_LLEB128(_word) \
     LFSR_DATA_LLEB128_(_word, (uint8_t[LFSR_LLEB128_DSIZE]){0})
@@ -1472,7 +1472,7 @@ typedef struct lfsr_attr {
         .cat=_cat})
 
 #define LFSR_ATTR(_tag, _delta, _data) \
-    lfsr_attr(_tag, _delta, *(lfsr_data_t[1]){_data})
+    (*(lfsr_attr_t[1]){lfsr_attr(_tag, _delta, _data)})
 
 static inline lfsr_attr_t lfsr_attr(
         lfsr_tag_t tag, lfsr_srid_t delta, lfsr_data_t data) {
@@ -1776,7 +1776,7 @@ typedef struct lfsr_ecksum {
 #define LFSR_ECKSUM_DSIZE (4+4)
 
 #define LFSR_DATA_ECKSUM_(_ecksum, _buffer) \
-    lfsr_data_fromecksum(_ecksum, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_fromecksum(_ecksum, _buffer)})
 
 #define LFSR_DATA_ECKSUM(_ecksum) \
     LFSR_DATA_ECKSUM_(_ecksum, (uint8_t[LFSR_ECKSUM_DSIZE]){0})
@@ -1833,7 +1833,7 @@ static int lfsr_data_readecksum(lfs_t *lfs, lfsr_data_t *data,
 #define LFSR_BPTR_DSIZE (4+5+4+4+4)
 
 #define LFSR_DATA_BPTR_(_bptr, _buffer) \
-    lfsr_data_frombptr(_bptr, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_frombptr(_bptr, _buffer)})
 
 #define LFSR_DATA_BPTR(_bptr) \
     LFSR_DATA_BPTR_(_bptr, (uint8_t[LFSR_BPTR_DSIZE]){0})
@@ -2045,7 +2045,7 @@ static inline bool lfsr_grm_isrm(const lfsr_grm_t *grm, lfsr_smid_t mid) {
 }
 
 #define LFSR_DATA_GRM_(_grm, _buffer) \
-    lfsr_data_fromgrm(_grm, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_fromgrm(_grm, _buffer)})
 
 #define LFSR_DATA_GRM(_grm) \
     LFSR_DATA_GRM_(_grm, (uint8_t[LFSR_GRM_DSIZE]){0})
@@ -4120,7 +4120,7 @@ static inline int lfsr_btree_cmp(
 #define LFSR_BRANCH_DSIZE (5+4+4)
 
 #define LFSR_DATA_BRANCH_(_branch, _buffer) \
-    lfsr_data_frombranch(_branch, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_frombranch(_branch, _buffer)})
 
 #define LFSR_DATA_BRANCH(_branch) \
     LFSR_DATA_BRANCH_(_branch, (uint8_t[LFSR_BRANCH_DSIZE]){0})
@@ -4193,7 +4193,7 @@ static int lfsr_data_readbranch(lfs_t *lfs, lfsr_data_t *data,
 #define LFSR_BTREE_DSIZE (5+LFSR_BRANCH_DSIZE)
 
 #define LFSR_DATA_BTREE_(_btree, _buffer) \
-    lfsr_data_frombtree(_btree, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_frombtree(_btree, _buffer)})
 
 #define LFSR_DATA_BTREE(_btree) \
     LFSR_DATA_BTREE_(_btree, (uint8_t[LFSR_BTREE_DSIZE]){0})
@@ -5264,7 +5264,7 @@ static inline int lfsr_shrub_cmp(
 #define LFSR_SHRUB_DSIZE (5+4)
 
 #define LFSR_DATA_SHRUB_(_rbyd, _buffer) \
-    lfsr_data_fromshrub(_rbyd, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_fromshrub(_rbyd, _buffer)})
 
 #define LFSR_DATA_SHRUB(_rbyd) \
     LFSR_DATA_SHRUB_(_rbyd, (uint8_t[LFSR_SHRUB_DSIZE]){0})
@@ -5469,7 +5469,7 @@ static inline bool lfsr_mptr_ismrootanchor(const lfsr_mptr_t *mptr) {
 #define LFSR_MPTR_DSIZE (5+5)
 
 #define LFSR_DATA_MPTR_(_mptr, _buffer) \
-    lfsr_data_frommptr(_mptr, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_frommptr(_mptr, _buffer)})
 
 #define LFSR_DATA_MPTR(_mptr) \
     LFSR_DATA_MPTR_(_mptr, (uint8_t[LFSR_MPTR_DSIZE]){0})
@@ -8064,7 +8064,7 @@ typedef struct lfsr_geometry {
 #define LFSR_GEOMETRY_DSIZE (4+5)
 
 #define LFSR_DATA_GEOMETRY_(_geometry, _buffer) \
-    lfsr_data_fromgeometry(_geometry, _buffer)
+    (*(lfsr_data_t[1]){lfsr_data_fromgeometry(_geometry, _buffer)})
 
 #define LFSR_DATA_GEOMETRY(_geometry) \
     LFSR_DATA_GEOMETRY_(_geometry, (uint8_t[LFSR_GEOMETRY_DSIZE]){0})