This uses a chunk of code from btrfs_read_dev_super() and creates
a function called btrfs_read_dev_one_super() so that next patch
can use it for scratch superblock.
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
[renamed bufhead to bh]
Signed-off-by: NDavid Sterba <dsterba@suse.com>