selftests/mm: skip uffd-wp-mremap if userfaultfd not available

selftests/mm: skip uffd-wp-mremap if userfaultfd not available

It's obvious that this should fail in that case, but still, save the
reader the effort of figuring out that they've run into this by just
SKIPping

Link: https://lkml.kernel.org/r/20250221-mm-selftests-v2-4-28c4d66383c5@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>