test/rcu: fix build for small number of cores

test/rcu: fix build for small number of cores

If the RTE_MAX_LCORE is less than 10, a compilation error is generated:
app/test/test_rcu_qsbr.c:234:10: error: comparison of integer
expressions of different signedness: ‘unsigned int’ and ‘int’
[-Werror=sign-compare]

The cause is (RTE_MAX_LCORE - 10) results in a negative value.

To fix, use rte_rand() to find a number between 0 and RTE_MAX_LCORE.

Fixes: b87089b0bb19 ("test/rcu: add API and functional tests")
Cc: stable@dpdk.org
Signed-off-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Dharmik Thakkar <dharmik.thakkar@arm.com>