userfaultfd: selftest: explicit failure if the SIGBUS test failed

Showing zero in the output isn't very self explanatory as a successful
result.  Show a more explicit error output if the test fails.

Link: http://lkml.kernel.org/r/20170802165145.22628-4-aarcange@redhat.comSigned-off-by: NAndrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Alexey Perevalov <a.perevalov@samsung.com>
Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/userfaultfd.c

@@ -987,7 +987,9 @@ static int userfaultfd_sig_test(void)
 		return 1;
 
 	printf("done.\n");
-	printf(" Signal test userfaults: %ld\n", userfaults);
+	if (userfaults)
+		fprintf(stderr, "Signal test failed, userfaults: %ld\n",
+			userfaults);
 	close(uffd);
 	return userfaults != 0;
 }