SUNRPC: Fix use of copy_to_user() in gss_pipe_upcall()

The gss_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.

Can rpc_pipefs actually retry a partially completed upcall read?  If
not, then gss_pipe_upcall() should punt any partial read, just like the
upcall logic in net/sunrpc/cache.c.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/auth_gss/auth_gss.c

@@ -472,16 +472,15 @@ gss_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;
-	ssize_t left;
+	size_t mlen = min(msg->len, buflen);
+	unsigned long left;
 
-	if (mlen > buflen)
-		mlen = buflen;
 	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;