ovl: support idmapped layers

Now that overlay is able to take a layers idmapping into account allow
overlay mounts to be created on top of idmapped mounts. Since NFS
doesn't support idmapped mounts we don't allow idmapped base layers in
combination with the nfs_export=on mount option.

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>
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 9a656a2..d4cc07f 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -874,10 +874,6 @@
 		pr_err("filesystem on '%s' not supported\n", name);
 		goto out_put;
-	if (is_idmapped_mnt(path->mnt)) {
-		pr_err("idmapped layers are currently not supported\n");
-		goto out_put;
-	}
 	if (!d_is_dir(path->dentry)) {
 		pr_err("'%s' not a directory\n", name);
 		goto out_put;