examples/ipsec-secgw: fix outbound codepath for single SA

examples/ipsec-secgw: fix outbound codepath for single SA

Looking at process_pkts_outbound_nosp() there seems few issues:
- accessing mbuf after it was freed
- invoking ipsec_outbound() for ipv4 packets only
- copying number of packets, but not the mbuf pointers itself

that patch provides fixes for that issues.

Fixes: 906257e965b7 ("examples/ipsec-secgw: support IPv6")
Cc: stable@dpdk.org
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>