trv: Added LFS3_t_NOSPC, avoid ENOSPC errors in traversals

This relaxes error encountered during lfs3_mtree_gc to _not_ propagate,
but instead just log a warning and prevent the relevant work from being
checked off during EOT.

The idea is this allows other work to make progress in low-space
conditions.

I originally meant to limit this to gbmap repopulations, to match the
behavior of lfs3_alloc_repopgbmap, but I think extending the idea to all
filesystem mutating operations makes sense (LFS3_T_MKCONSISTENT +
LFS3_T_REPOPGBMAP + LFS3_T_COMPACTMETA).

---

To avoid incorrectly marking traversal work as completed, we need to
track if we hit any ENOSPC errors, thus the new LFS3_t_NOSPC flag:

  LFS3_t_NOSPC  0x00800000  Optional gc work ran out of space

Not the happiest just throwing flags at problems, but I can't think of a
better solution at the moment.

This doesn't differentiate between ENOSPC errors during the different
types of work, but in theory if we're hitting ENOSPC errors whatever
work returns the error is a toss-up anyways.

---

Adds a bit of code:

                 code          stack          ctx
  before:       37208           2352          688
  after:        37248 (+0.1%)   2352 (+0.0%)  688 (+0.0%)

                 code          stack          ctx
  gbmap before: 40120           2368          856
  gbmap after:  40204 (+0.2%)   2368 (+0.0%)  856 (+0.0%)

lfs3.h

@@ -342,12 +342,13 @@ enum lfs3_btype {
 // internally used flags, don't use these
 #define LFS3_t_TYPE     0xf0000000  // The traversal's type
 #define LFS3_t_TSTATE   0x000f0000  // The current traversal state
-#define LFS3_t_BTYPE    0x00f00000  // The current block type
+#define LFS3_t_BTYPE    0x00700000  // The current block type
 #define LFS3_t_ZOMBIE   0x08000000  // File has been removed
 #define LFS3_t_DIRTY    0x04000000  // Filesystem modified outside traversal
 #define LFS3_t_MUTATED  0x02000000  // Filesystem modified during traversal
 #define LFS3_t_CKPOINTED \
                         0x01000000  // Filesystem ckpointed during traversal
+#define LFS3_t_NOSPC    0x00800000  // Optional gc work ran out of space
 
 // GC flags
 #ifndef LFS3_RDONLY