Split crystal_thresh into crystal_thresh + fragment_thresh

So now crystal_thresh only controls when fragments are compacted into
blocks, while fragment_thresh controls when blocks are broken into
fragments. Setting fragment_thresh=-1 will follow crystal_thresh and
keeps the previous behavior.

These were already two separate pieces of logic, so it makes sense to
provide two separate knobs for tuning.

Setting fragment_thresh lower than crystal_thresh has some potential to
reduce hysteresis in cases where random writes push blocks close to
crystal_thresh. It will be interesting to explore this more when
benchmarking.

---

The additional config option adds a bit of code/ctx, but hopefully that
will go away in the future config rework:

           code          stack          ctx
  before: 35584           2480          636
  after:  35600 (+0.0%)   2480 (+0.0%)  640 (+0.6%)

runners/bench_runner.h

@@ -120,6 +120,7 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
     BENCH_DEFINE(INLINE_SIZE,        BLOCK_SIZE/4                           ) \
     BENCH_DEFINE(FRAGMENT_SIZE,      BLOCK_SIZE/8                           ) \
     BENCH_DEFINE(CRYSTAL_THRESH,     BLOCK_SIZE/8                           ) \
+    BENCH_DEFINE(FRAGMENT_THRESH,    -1                                     ) \
     BENCH_DEFINE(ERASE_VALUE,        0xff                                   ) \
     BENCH_DEFINE(ERASE_CYCLES,       0                                      ) \
     BENCH_DEFINE(BADBLOCK_BEHAVIOR,  LFS_EMUBD_BADBLOCK_PROGERROR           ) \
@@ -148,7 +149,8 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
     .gc_compact_thresh  = GC_COMPACT_THRESH,    \
     .inline_size        = INLINE_SIZE,          \
     .fragment_size      = FRAGMENT_SIZE,        \
-    .crystal_thresh     = CRYSTAL_THRESH,
+    .crystal_thresh     = CRYSTAL_THRESH,       \
+    .fragment_thresh    = FRAGMENT_THRESH,
 
 #ifdef LFS_GC
 #define BENCH_GC_CFG                            \