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>

tools/testing/selftests/mm/run_vmtests.sh

@@ -353,7 +353,7 @@ echo "$nr_hugepgs" > /proc/sys/vm/nr_hugepages
 
 CATEGORY="compaction" run_test ./compaction_test
 
-if command -v sudo &> /dev/null;
+if command -v sudo &> /dev/null && sudo -u nobody ls ./on-fault-limit >/dev/null;
 then
 	CATEGORY="mlock" run_test sudo -u nobody ./on-fault-limit
 else