bcachefs: minor lru fsck fixes

check_lru_key() wasn't using write buffer updates for deleting bad lru
entries - dating from before the lru btree used the btree write buffer.

And when possibly flushing the btree write buffer (to make sure we're
seeing a real inconsistency), we need to be using the modern
bch2_btree_write_buffer_maybe_flush().

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