userfaultfd/selftests: hint the test runner on required privilege

Now userfaultfd test program requires either root or ptrace privilege due
to the signal/event tests.  When UFFDIO_API failed, hint the test runner
about this fact verbosely.

Link: https://lkml.kernel.org/r/20201208024709.7701-4-peterx@redhat.comSigned-off-by: Peter Xu <peterx@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/userfaultfd.c

@@ -794,7 +794,8 @@ static int userfaultfd_open(int features)
 	uffdio_api.api = UFFD_API;
 	uffdio_api.features = features;
 	if (ioctl(uffd, UFFDIO_API, &uffdio_api)) {
-		fprintf(stderr, "UFFDIO_API\n");
+		fprintf(stderr, "UFFDIO_API failed.\nPlease make sure to "
+			"run with either root or ptrace capability.\n");
 		return 1;
 	}
 	if (uffdio_api.api != UFFD_API) {