bcachefs: don't use rht_bucket() in btree_key_cache_scan()

rht_bucket() does strange complicated things when a rehash is in
progress.

Instead, just skip scanning when a rehash is in progress: scanning is
going to be more expensive (many more empty slots to cover), and some
sort of infinite loop is being observed

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