Relaxed fragment_size limit from block_size/8 -> block_size/4

The concern with block_size/4 is that it limits fragments to a single
fragment per-block. But while this may be inefficient, it's technically
not wrong, and may still work with other metadata (bptrs, file names,
uattrs, etc) taking up the remaining space.

This deserves benchmarking, but even if this ends up being a terrible
configuration, we should just discourage this via good defaults and
documentation.

lfs.h

@@ -286,7 +286,7 @@ struct lfs_config {
 
     // Maximum size of a non-block B-tree leaf in bytes. Smaller values may
     // make small random-writes cheaper, but increase metadata overhead. Must
-    // be <= block_size/8.
+    // be <= block_size/4.
     lfs_size_t fragment_size;
 
     // Threshold for compacting multiple fragments into a block. Smaller