selftests/mm: report SKIP in pfnmap if a check fails

selftests/mm: report SKIP in pfnmap if a check fails

pfnmap currently checks the target file in FIXTURE_SETUP(pfnmap), meaning
once for every test, and skips the test if any check fails.

The target file is the same for every test so this is a little overkill.
More importantly, this approach means that the whole suite will report
PASS even if all the tests are skipped because kernel configuration (e.g.
CONFIG_STRICT_DEVMEM=y) prevented /dev/mem from being mapped, for
instance.

Let's ensure that KSFT_SKIP is returned as exit code if any check fails by
performing the checks in pfnmap_init(), run once.  That function also
takes care of finding the offset of the pages to be mapped and saves it in
a global.  The file is now opened only once and the fd saved in a global,
but it is still mapped/unmapped for every test, as some of them modify the
mapping.

Link: https://lkml.kernel.org/r/20260122170224.4056513-10-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Usama Anjum <Usama.Anjum@arm.com>
Cc: wang lian <lianux.mm@gmail.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>