btrfs: pass btrfs_device to btrfs_scratch_superblocks()

Replace the two parameters bdev and name by one that can be used to get
them both.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>