[NET]: Fix error reporting in sys_socketpair().

If either of the two sock_alloc_fd() calls fail, we
forget to update 'err' and thus we'll erroneously
return zero in these cases.

Based upon a report and patch from Rich Paul, and
commentary from Chuck Ebbert.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/socket.c

@@ -1250,11 +1250,14 @@ asmlinkage long sys_socketpair(int family, int type, int protocol,
 		goto out_release_both;
 
 	fd1 = sock_alloc_fd(&newfile1);
-	if (unlikely(fd1 < 0))
+	if (unlikely(fd1 < 0)) {
+		err = fd1;
 		goto out_release_both;
+	}
 
 	fd2 = sock_alloc_fd(&newfile2);
 	if (unlikely(fd2 < 0)) {
+		err = fd2;
 		put_filp(newfile1);
 		put_unused_fd(fd1);
 		goto out_release_both;