examples/ipsec-secgw: make number of buffers dynamic

examples/ipsec-secgw: make number of buffers dynamic

Make number of buffers in a pool nb_mbuf_in_pool dependent on number
of ports, cores and crypto queues. Add command line option -s which
when used overrides dynamic calculation of number of buffers in a pool.

Signed-off-by: Anoob Joseph <anoobj@marvell.com>
Signed-off-by: Lukasz Bartosik <lbartosik@marvell.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>