btrfs: add helper function find_fsid_by_disk

In preparation for adding support to mount multiple single-disk
btrfs filesystems with the same FSID, wrap find_fsid() into
find_fsid_by_disk().

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>