Bump default inline_max up to 1/4 block size

As noted by amgross, the current inline_max default (when littlefs
switches from inline files to CTZ skip-lists) does not match the
theoretical value in DESIGN.md.

The reason for this is 1/8 was chosen as a safer default during
development, due to concerns that 1/4 + mdirs splitting at 1/2 would
lead to poor distribution of large inline files in mdirs.

However, two things have happened since then:

1. Experiments have show "wasted" mdir space is less of a concern than
   initially thought. Extra mdir space contributes to logging, delays
   mdir compaction, and can overall lead to better performance.

2. inline_size was added as a configuration option, so if 1/4 is
   problematic users can always override it.

So bumping this back up to 1/4 may make sense.

lfs.c

@@ -4331,7 +4331,7 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     LFS_ASSERT(lfs->cfg->inline_max == (lfs_size_t)-1
             || lfs->cfg->inline_max <= ((lfs->cfg->metadata_max)
                 ? lfs->cfg->metadata_max
-                : lfs->cfg->block_size)/8);
+                : lfs->cfg->block_size)/4);
     lfs->inline_max = lfs->cfg->inline_max;
     if (lfs->inline_max == (lfs_size_t)-1) {
         lfs->inline_max = 0;
@@ -4342,7 +4342,7 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
                     lfs->attr_max,
                     ((lfs->cfg->metadata_max)
                         ? lfs->cfg->metadata_max
-                        : lfs->cfg->block_size)/8));
+                        : lfs->cfg->block_size)/4));
     }
 
     // setup default state

lfs.h

@@ -277,7 +277,7 @@ struct lfs_config {
     // Optional upper limit on inlined files in bytes. Inlined files live in
     // metadata and decrease storage requirements, but may be limited to
     // improve metadata-related performance. Must be <= cache_size, <=
-    // attr_max, and <= block_size/8. Defaults to the largest possible
+    // attr_max, and <= block_size/4. Defaults to the largest possible
     // inline_max when zero.
     //
     // Set to -1 to disable inlined files.