btrfs: store a pointer to a btrfs_bio in struct btrfs_bio_ctrl

The bio in struct btrfs_bio_ctrl must be a btrfs_bio, so store a pointer
to the btrfs_bio for better type checking.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>