btrfs: allow idmapped mkdir inode op

Enable btrfs_mkdir() 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

@@ -6926,7 +6926,7 @@ static int btrfs_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
 	if (err)
 		goto out_fail;
 
-	inode = btrfs_new_inode(trans, root, &init_user_ns, dir,
+	inode = btrfs_new_inode(trans, root, mnt_userns, dir,
 			dentry->d_name.name, dentry->d_name.len,
 			btrfs_ino(BTRFS_I(dir)), objectid,
 			S_IFDIR | mode, &index);