selftests/mm: skip uffd-stress if userfaultfd not available

selftests/mm: skip uffd-stress if userfaultfd not available

It's pretty obvious that the test wouldn't work if you don't have the
feature enabled.  But, it's still useful to SKIP instead of failing so the
reader can immediately tell that this is the reason why.

Link: https://lkml.kernel.org/r/20250221-mm-selftests-v2-3-28c4d66383c5@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>