Added lfs_memlen, replacing lfsr_gdelta_size

This drops the last lfsr_gdelta_* function, which were really just a
bunch of somewhat-quirky mem operations.

Moving this to lfs_util.h also allows users to override it with
hardware-specific tricks, though I think hardware tricks for lfs_memlen
will be quite rare. Reverse-order memory optimizations are pretty
uncommon...

This could also be done with a theoretical memrcchr, if one existed:

  p = memrcchr(buffer, 0, size);
  return (p) ? p - buffer : 0;

But I figured this use case is so niche we might as well just limit it
to c=0 (and avoid questions about memrchr).

No code changes.

lfs.c

@@ -6818,17 +6818,6 @@ static void lfsr_fs_mkdirty(lfs_t *lfs) {
 
 /// Global-state things ///
 
-static inline lfs_size_t lfsr_gdelta_size(
-        const uint8_t *gdelta, lfs_size_t size) {
-    // truncate based on number of trailing zeros
-    while (size > 0 && gdelta[size-1] == 0) {
-        size -= 1;
-    }
-
-    return size;
-}
-
-
 // gcksum (global checksum) things
 
 // cubing the gcksum prevents trivial gcksumdeltas
@@ -6886,7 +6875,7 @@ static lfsr_data_t lfsr_data_fromgrm(const lfsr_grm_t *grm,
         d += d_;
     }
 
-    return LFSR_DATA_BUF(buffer, lfsr_gdelta_size(buffer, LFSR_GRM_DSIZE));
+    return LFSR_DATA_BUF(buffer, lfs_memlen(buffer, LFSR_GRM_DSIZE));
 }
 
 // required by lfsr_data_readgrm
@@ -6922,6 +6911,7 @@ static int lfsr_data_readgrm(lfs_t *lfs, lfsr_data_t *data,
     return 0;
 }
 
+
 // some mdir-related gstate things we need
 static void lfsr_fs_flushgdelta(lfs_t *lfs) {
     // zero any pending gdeltas
@@ -6960,7 +6950,7 @@ static int lfsr_rbyd_appendgdelta(lfs_t *lfs, lfsr_rbyd_t *rbyd) {
     lfs_memxor(grmdelta_, lfs->grm_p, LFSR_GRM_DSIZE);
     lfs_memxor(grmdelta_, lfs->grm_d, LFSR_GRM_DSIZE);
 
-    if (lfsr_gdelta_size(grmdelta_, LFSR_GRM_DSIZE) != 0) {
+    if (lfs_memlen(grmdelta_, LFSR_GRM_DSIZE) != 0) {
         // make sure to xor any existing delta
         lfsr_data_t data;
         int err = lfsr_rbyd_lookup(lfs, rbyd, -1, LFSR_TAG_GRMDELTA,
@@ -6982,7 +6972,7 @@ static int lfsr_rbyd_appendgdelta(lfs_t *lfs, lfsr_rbyd_t *rbyd) {
         lfs_memxor(grmdelta_, grmdelta, LFSR_GRM_DSIZE);
 
         // append to our rbyd, replacing any existing delta
-        lfs_size_t size = lfsr_gdelta_size(grmdelta_, LFSR_GRM_DSIZE);
+        lfs_size_t size = lfs_memlen(grmdelta_, LFSR_GRM_DSIZE);
         err = lfsr_rbyd_appendrat(lfs, rbyd, -1, LFSR_RAT(
                 // opportunistically remove this tag if delta is all zero
                 (size == 0)

lfs_util.h

@@ -473,6 +473,16 @@ static inline void *lfs_memcchr(const void *a, int c, size_t size) {
     return NULL;
 }
 
+// Find the minimum length that includes all non-zero bytes
+static inline size_t lfs_memlen(const void *a, size_t size) {
+    const uint8_t *a_ = a;
+    while (size > 0 && a_[size-1] == 0) {
+        size -= 1;
+    }
+
+    return size;
+}
+
 // Xor n bytes from b into a
 static inline void *lfs_memxor(
         void *restrict a, const void *restrict b, size_t size) {