examples/ipsec-secgw: fix session mempool initialisation

examples/ipsec-secgw: fix session mempool initialisation

Creation of a session mempool may fail in the case of a single lcore
and a low number of SA.

In case when there is only one lcore and number of configures SA less
then (0.5 * CDEV_MP_CACHE_SZ) then creation of the mempool fails with
EINVAL. This is because the number of requested items is less than
(cache size * CDEV_MP_CACHE_MULTIPLIER).

Hence, the number of elements in mempool is increased in such cases.

Fixes: e30b2833c47c ("security: update session create API")

Signed-off-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>