nfs: don't lose MS_SYNCHRONOUS on remount of noac mount

On a remount, the VFS layer will clear the MS_SYNCHRONOUS bit on the
assumption that the flags on the mount syscall will have it set if the
remounted fs is supposed to keep it.

In the case of "noac" though, MS_SYNCHRONOUS is implied. A remount of
such a mount will lose the MS_SYNCHRONOUS flag since "sync" isn't part
of the mount options.
Reported-by: Max Matveev <makc@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/super.c

@@ -1977,6 +1977,15 @@ nfs_remount(struct super_block *sb, int *flags, char *raw_data)
 	if (error < 0)
 		goto out;
 
+	/*
+	 * noac is a special case. It implies -o sync, but that's not
+	 * necessarily reflected in the mtab options. do_remount_sb
+	 * will clear MS_SYNCHRONOUS if -o sync wasn't specified in the
+	 * remount options, so we have to explicitly reset it.
+	 */
+	if (data->flags & NFS_MOUNT_NOAC)
+		*flags |= MS_SYNCHRONOUS;
+
 	/* compare new mount options with old ones */
 	error = nfs_compare_remount_data(nfss, data);
 out: