selftests/mm: skip guard_regions.uffd tests when uffd is not present

selftests/mm: skip guard_regions.uffd tests when uffd is not present

Patch series "Skip mm selftests instead when kernel features are not
present", v2.

Two guard_regions tests on userfaultfd fail when userfaultfd is not
present.  Skip them instead.

hugevm test reads kernel config to get page table level information and
fails when neither /proc/config.gz nor /boot/config-* is present.  Skip it
instead.

This patch (of 2):

When userfaultfd is not compiled into kernel, userfaultfd() returns -1,
causing guard_regions.uffd tests to fail.  Skip the tests instead.

Link: https://lkml.kernel.org/r/20250516132938.356627-1-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20250516132938.356627-2-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Cc: Adam Sindelar <adam@wowsignal.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>