selftests/mm: report errno when things fail in gup_longterm

selftests/mm: report errno when things fail in gup_longterm

Patch series "selftests/mm: Some cleanups from trying to run them", v2.

I never had much luck running mm selftests so I spent a couple of hours
digging into why.

Looks like most of the reason is missing SKIP checks, so this series is
just adding a bunch of those that I found.  I did not do anything like all
of them, just the ones I spotted in gup_longterm, gup_test, mmap,
userfaultfd and memfd_secret.

It's a bit unfortunate to have to skip those tests when ftruncate() fails,
but I don't have time to dig deep enough into it to actually make them
pass - I observed these issues on both 9p and virtiofs.  Probably it
requires digging into the filesystem implementation

(An alternative might just be to mount a tmpfs in the test script).

I am also seeing some failures to allocate hugetlb pages in uffd-mp-mremap
that I have not had time to fully understand, you can see those here:

https://gist.github.com/bjackman/af74c3a6e60975e6ff0d760cba1e05d2#file-userfaultfd-log

This patch (of 9):

Just reporting failure doesn't tell you what went wrong.  This can fail in
different ways so report errno to help the reader get started debugging.

Link: https://lkml.kernel.org/r/20250221-mm-selftests-v2-0-28c4d66383c5@google.com
Link: https://lkml.kernel.org/r/20250221-mm-selftests-v2-1-28c4d66383c5@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>