btrfs: hold a ref on the root in create_subvol

We're creating the new root here, but we should hold the ref until after
we've initialized the inode for it.

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