selftests/mm: fix exit code in pagemap_ioctl

Make sure pagemap_ioctl exits with an appropriate value:

* If the tests are run, call ksft_finished() to report the right
status instead of reporting PASS unconditionally.

* Report SKIP if userfaultfd isn't available (in line with other
tests)

* Report FAIL if we failed to open /proc/self/pagemap, as this file
has been added a long time ago and doesn't depend on any CONFIG
option (returning -EINVAL from main() is meaningless)

Link: https://lkml.kernel.org/r/20260122170224.4056513-9-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: SeongJae Park <sj@kernel.org>
Reviewed-by: wang lian <lianux.mm@gmail.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Usama Anjum <Usama.Anjum@arm.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>