bcachefs: Kill opts.buckets_nouse

Now explicitly allocate and free the buckets_nouse bitmap - this is
going to be used for online fsck.

To go RW when we haven't check allocations, we'll do a much slimmed down
version that just initializes the buckets_nouse bitmaps.

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