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.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.