bcachefs: Refactor bch2_check_alloc_to_lru_ref()

This code was somewhat convoluted - because originally bch2_lru_set()
could modify the LRU index if there was a collision.

That's no longer the case, so the "create LRU entry" path has no reason
to update the alloc key, so we can separate the handling of the two fsck
errors.

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