trv: Reverted LFS3_t_NOSPC, forward gbmap repop errors

Note: This affects the blocking lfs3_alloc_repopgbmap as well as
incremental gc/traversal repopulations. Now all repop attempts return
LFS3_ERR_NOSPC when we don't have space for the gbmap, motivation below.

This reverts the previous LFS3_t_NOSPC soft error, in which traversals
were allowed to continue some gc/traversal work when encountering
LFS3_ERR_NOSPC. This results in a simpler implementation and fewer error
cases to worry about.

Observation/motivation:

- The main motivation is noticing that when we're in low-space
  conditions, we just start spamming gbmap repops even if they all fail.

  That's really not great! We might as well just mark the flash as dead
  if we're going to start spamming erases!

  At least with an error the user can call rmgbmap to try to make
  progress.

- If we're in a low-space condition, something else will probably return
  LFS3_ERR_NOSPC anyways. Might as well report this early and simplify
  our system.

- It's a simpler model, and littlefs3 is already much more complicated
  than littlefs2. Maybe we should lean more towards a simpler system
  at the cost of some niche optimizations.

---

This had the side-effect of causing more lfs3_alloc_ckpoints to return
errors during testing, which revealed a bug in our uz/uzd_fuzz tests:

- We weren't flushing after writes to the opened RDWR files, which could
  cause delayed errors to occur during the later read checks in the
  test.

  Fortunately LFS3_O_FLUSH provides a quick and easy fix!

  Note we _don't_ adopt this in all uz/uzd_fuzz tests, only those that
  error. It's good to test both with and without LFS3_O_FLUSH to test
  that read-flushing also works under stress.

Saves a bit of code:

                 code          stack          ctx
  before:       37260           2352          688
  after:        37220 (-0.1%)   2352 (+0.0%)  688 (+0.0%)

                 code          stack          ctx
  gbmap before: 40220           2368          856
  gbmap after:  40184 (-0.1%)   2368 (+0.0%)  856 (+0.0%)

lfs3.h

@@ -342,13 +342,12 @@ 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    0x00700000  // The current block type
+#define LFS3_t_BTYPE    0x00f00000  // 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