btrfs: print correct subvol num if active swapfile prevents deletion

Fix the error message in btrfs_delete_subvolume() if we can't delete a
subvolume because it has an active swapfile: we were printing the number
of the parent rather than the target.

Fixes: 60021bd ("btrfs: prevent subvol with swapfile from being deleted")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Mark Harmstone <mark@harmstone.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: maharmstone

fs/btrfs/inode.c

@@ -4779,7 +4779,7 @@ int btrfs_delete_subvolume(struct btrfs_inode *dir, struct dentry *dentry)
 		spin_unlock(&dest->root_item_lock);
 		btrfs_warn(fs_info,
 			   "attempt to delete subvolume %llu with active swapfile",
-			   btrfs_root_id(root));
+			   btrfs_root_id(dest));
 		ret = -EPERM;
 		goto out_up_write;
 	}