bcachefs: add check for missing fragmentation in check_alloc_to_lru_ref()

We need to make sure we're not missing any fragmenation entries in the
LRU BTREE after repairing ALLOC BTREE

Also, use the new bch2_btree_write_buffer_maybe_flush() helper; this was
only working without it before since bucket invalidation (usually)
wasn't happening while fsck was running.

Co-developed-by: Daniel Hill <daniel@gluo.nz>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>