ovl: handle idmappings for layer fileattrs

Take the upper mount's idmapping into account when setting fileattrs on
the upper layer. This is needed to support idmapped base layers with
overlay.

Cc: <linux-unionfs@vger.kernel.org>
Tested-by: Giuseppe Scrivano <gscrivan@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -537,7 +537,7 @@ int ovl_real_fileattr_set(struct path *realpath, struct fileattr *fa)
 	if (err)
 		return err;
 
-	return vfs_fileattr_set(&init_user_ns, realpath->dentry, fa);
+	return vfs_fileattr_set(mnt_user_ns(realpath->mnt), realpath->dentry, fa);
 }
 
 int ovl_fileattr_set(struct user_namespace *mnt_userns,