btrfs: allow idmapped tmpfile inode op

Enable btrfs_tmpfile() to handle idmapped mounts. This is just a matter
of passing down the mount's userns.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -10429,7 +10429,7 @@ static int btrfs_tmpfile(struct user_namespace *mnt_userns, struct inode *dir,
 	if (ret)
 		goto out;
 
-	inode = btrfs_new_inode(trans, root, &init_user_ns, dir, NULL, 0,
+	inode = btrfs_new_inode(trans, root, mnt_userns, dir, NULL, 0,
 			btrfs_ino(BTRFS_I(dir)), objectid, mode, &index);
 	if (IS_ERR(inode)) {
 		ret = PTR_ERR(inode);