selftests/mm: skip mlock tests if nobody user can't read it

If running from a directory that can't be read by unprivileged users,
executing on-fault-test via the nobody user will fail.

The kselftest build does give the file the correct permissions, but after
being installed it might be in a directory without global execute
permissions.

Since the script can't safely fix that, just skip if it happens.  Note
that the stderr of the `ls` command is unfiltered meaning the user sees a
"permission denied" error that can help inform them why the test was
skipped.

Link: https://lkml.kernel.org/r/20250311-mm-selftests-v4-11-dec210a658f5@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 <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>