examples/ipsec-secgw: fix pool usage for security session

examples/ipsec-secgw: fix pool usage for security session

Currently, two separate mempools are being used for creating crypto
sessions and its private data.
crypto sessions are created and initialized separately, so a separate
mempool is passed to each API, but in case of security sessions, where
only one API create and initialize the private data as well.
So if session mempool is passed to create a security session, the
mempool element size is not sufficient enough to hold the private
data as well.
As a perfect solution, the security session create API should take 2
mempools for header and private data and initiatlize accordingly,
but that would mean an API breakage, which will be done in the next
release cycle. So introducing this patch as a workaround to resolve this
issue.

Fixes: 261bbff75e34 ("examples: use separate crypto session mempools")
Cc: stable@dpdk.org
Signed-off-by: Akhil Goyal <akhil.goyal@nxp.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>