selftests/mm: ensure uffd-wp-mremap gets pages of each size

This test allocates a page of every available size and doesn't have any
SKIP logic if the allocation fails.  So, ensure it's available and skip
the test if we can't do so.

Link: https://lkml.kernel.org/r/20250311-mm-selftests-v4-10-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>