switch create_mnt_ns() to saner calling conventions, fix double mntput() in nfs

Life is much saner if create_mnt_ns(mnt) drops mnt in case of error...
Switch it to such calling conventions, switch callers, fix double mntput() in
fs/nfs/super.c one.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index 480b3b6..46d69f38 100644
--- a/fs/nfs/super.c
+++ b/fs/nfs/super.c
@@ -2794,22 +2794,21 @@
 	int ret;
 
 	ns_private = create_mnt_ns(root_mnt);
-	ret = PTR_ERR(ns_private);
 	if (IS_ERR(ns_private))
-		goto out_mntput;
+		return ERR_CAST(ns_private);
 
 	ret = nfs_referral_loop_protect();
-	if (ret != 0)
-		goto out_put_mnt_ns;
+	if (ret == 0) {
+		ret = vfs_path_lookup(root_mnt->mnt_root, root_mnt,
+				export_path, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT,
+				&path);
+		nfs_referral_loop_unprotect();
+	}
 
-	ret = vfs_path_lookup(root_mnt->mnt_root, root_mnt,
-			export_path, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &path);
-
-	nfs_referral_loop_unprotect();
 	put_mnt_ns(ns_private);
 
 	if (ret != 0)
-		goto out_err;
+		return ERR_PTR(ret);
 
 	s = path.mnt->mnt_sb;
 	atomic_inc(&s->s_active);
@@ -2818,12 +2817,6 @@
 	path_put(&path);
 	down_write(&s->s_umount);
 	return dentry;
-out_put_mnt_ns:
-	put_mnt_ns(ns_private);
-out_mntput:
-	mntput(root_mnt);
-out_err:
-	return ERR_PTR(ret);
 }
 
 static struct dentry *nfs4_try_mount(int flags, const char *dev_name,