userfaultfd: selftest: add tests for UFFD_FEATURE_SIGBUS feature

userfaultfd: selftest: add tests for UFFD_FEATURE_SIGBUS feature

Add tests for UFFD_FEATURE_SIGBUS feature.  The tests will verify signal
delivery instead of userfault events.  Also, test use of UFFDIO_COPY to
allocate memory and retry accessing monitored area after signal
delivery.

Also fix a bug in uffd_poll_thread() where 'uffd' is leaked.

Link: http://lkml.kernel.org/r/1501552446-748335-3-git-send-email-prakash.sangappa@oracle.com
Signed-off-by: Prakash Sangappa <prakash.sangappa@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Andrea Arcangeli <aarcange@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>