test/rcu: increase size of core numbers

num_cores is of type uint8_t. This results in the following
compilation error.

test_rcu_qsbr_perf.c:649:16: error: comparison is always false
due to limited range of data type [-Werror=type-limits]
  if (num_cores >= RTE_MAX_LCORE) {
                ^~

RTE_MAX_LCORE is set to 256 for armv8 config.

Fixes: e6a14121f4ae ("test/rcu: remove arbitrary limit on max core count")
Cc: stable@dpdk.org
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>