selftests/mm/uffd: rename nr_cpus -> nr_threads

selftests/mm/uffd: rename nr_cpus -> nr_threads

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/20250221-mm-selftests-v2-5-28c4d66383c5@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>