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.com


Signed-off-by: Andrea 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>