btrfs: hold ref on root in btrfs_ioctl_default_subvol

We look up an arbitrary fs root here, we need to hold a ref on the root
for the duration.

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