selftests/mm/uffd: rename nr_cpus -> nr_parallel

selftests/mm/uffd: rename nr_cpus -> nr_parallel

A later commit will bound this variable so it no longer necessarily
matches the number of CPUs.  Rename it appropriately.

Link: https://lkml.kernel.org/r/20250311-mm-selftests-v4-4-dec210a658f5@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: 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>