ovl: handle idmappings for layer lookup

Make the two places where lookup helpers can be called either on lower
or upper layers take the mount's idmapping into account. To this end we
pass down the mount in struct ovl_lookup_data. It can later also be used
to construct struct path for various other helpers. 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/export.c

@@ -391,6 +391,11 @@ static struct dentry *ovl_lookup_real_one(struct dentry *connected,
 	 * pointer because we hold no lock on the real dentry.
 	 */
 	take_dentry_name_snapshot(&name, real);
+	/*
+	 * No mnt_userns handling here: it's an internal lookup.  Could skip
+	 * permission checking altogether, but for now just use non-mnt_userns
+	 * transformed ids.
+	 */
 	this = lookup_one_len(name.name.name, connected, name.name.len);
 	release_dentry_name_snapshot(&name);
 	err = PTR_ERR(this);