bcachefs: bch2_bucket_ref_update()

If we hit an inconsistency when updating allocation information, we
don't want to fail the update if it's for a deletion - only if it's for
a new key.

Rename check_bucket_ref() -> bucket_ref_update() so we can centralize
the logic to do this.

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