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>

fs/btrfs/super.c

@@ -843,10 +843,8 @@ static struct dentry *mount_subvol(const char *subvol_name, int flags,
 		return ERR_CAST(mnt);
 
 	ns_private = create_mnt_ns(mnt);
-	if (IS_ERR(ns_private)) {
-		mntput(mnt);
+	if (IS_ERR(ns_private))
 		return ERR_CAST(ns_private);
-	}
 
 	/*
 	 * This will trigger the automount of the subvol so we can just

fs/namespace.c

@@ -2483,6 +2483,8 @@ struct mnt_namespace *create_mnt_ns(struct vfsmount *mnt)
 		__mnt_make_longterm(mnt);
 		new_ns->root = mnt;
 		list_add(&new_ns->list, &new_ns->root->mnt_list);
+	} else {
+		mntput(mnt);
 	}
 	return new_ns;
 }

fs/nfs/super.c

@@ -2794,22 +2794,21 @@ static struct dentry *nfs_follow_remote_path(struct vfsmount *root_mnt,
 	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;
-
-	ret = vfs_path_lookup(root_mnt->mnt_root, root_mnt,
-			export_path, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT, &path);
+	if (ret == 0) {
+		ret = vfs_path_lookup(root_mnt->mnt_root, root_mnt,
+				export_path, LOOKUP_FOLLOW|LOOKUP_AUTOMOUNT,
+				&path);
+		nfs_referral_loop_unprotect();
+	}
 
-	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 @@ static struct dentry *nfs_follow_remote_path(struct vfsmount *root_mnt,
 	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,