NFS: Fix use of copy_to_user() in idmap_pipe_upcall

The idmap_pipe_upcall() function expects the copy_to_user() function to
return a negative error value if the call fails, but copy_to_user()
returns an unsigned long number of bytes that couldn't be copied.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/idmap.c

@@ -358,17 +358,15 @@ idmap_pipe_upcall(struct file *filp, struct rpc_pipe_msg *msg,
 		  char __user *dst, size_t buflen)
 {
 	char *data = (char *)msg->data + msg->copied;
-	ssize_t mlen = msg->len - msg->copied;
-	ssize_t left;
-
-	if (mlen > buflen)
-		mlen = buflen;
+	size_t mlen = min(msg->len, buflen);
+	unsigned long left;
 
 	left = copy_to_user(dst, data, mlen);
-	if (left < 0) {
-		msg->errno = left;
-		return left;
+	if (left == mlen) {
+		msg->errno = -EFAULT;
+		return -EFAULT;
 	}
+
 	mlen -= left;
 	msg->copied += mlen;
 	msg->errno = 0;