test/rcu: fix build with low core count

test/rcu: fix build with low core count

When RTE_MAX_LCORE value is small, following compiler errors
are observed.

../app/test/test_rcu_qsbr.c:296:54: error: iteration 2 invokes
undefined behavior [-Werror=aggressive-loop-optimizations]

../app/test/test_rcu_qsbr.c:315:55: error: array subscript is above
array bounds [-Werror=array-bounds]

Fixes: b87089b0bb19 ("test/rcu: add API and functional tests")
Cc: stable@dpdk.org
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Tested-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>
Reviewed-by: Lukasz Wojciechowski <l.wojciechow@partner.samsung.com>