bcachefs: Kill alloc_v4.fragmentation_lru

The fragmentation_lru field hasn't been needed since we reworked the LRU
btrees to use the btree write buffer; previously it was used to resolve
collisions, but the revised LRU btree uses the backpointer (the bucket)
as part of the key.

It should have been deleted at the time of the LRU rework; since it
wasn't, that left places for bugs to hide, in check/repair.

This fixes LRU fsck on a filesystem image helpfully provided by a user
who disappeared before I could get his name for the reported-by.

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