bcachefs: Improve bch2_check_alloc_info()

bcachefs: Improve bch2_check_alloc_info()

This factors out a new helper from bch2_dev_freespace_init(),
bch2_get_key_or_hole(), and uses it in bch2_check_alloc_info(): we're
now able to process holes in the alloc btree as ranges, instead of one
bucket at a time.

This will improve fsck performance on new filesystems, or filesystems
where not every bucket has been used yet.

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