bcachefs: s/BCH_LRU_FRAGMENTATION_START/BCH_LRU_BUCKET_FRAGMENTATION/

bcachefs: s/BCH_LRU_FRAGMENTATION_START/BCH_LRU_BUCKET_FRAGMENTATION/

FRAGMENTATION_START was incorrect, there's currently only one
fragmentation LRU (at the end of the reserved bits for LRU type), and
we're getting ready to add a stripe fragmentation lru - so give it a
better name.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>