This is one of the last steps in getting rid of the main in-memory
bucket array.

This changes bch2_dev_usage_update() to take bkey_alloc_unpacked instead
of bucket_mark, and for the places where we are in fact working with
bucket_mark and don't have bkey_alloc_unpacked, we add a wrapper that
takes bucket_mark and converts to bkey_alloc_unpacked.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>