examples/ipsec-secgw: clean SA structure

examples/ipsec-secgw: clean SA structure

Cleanup ipsec_sa structure by removing every field that is already in
the rte_ipsec_session structure:
 * cryptodev/security session union
 * action type
 * offload flags
 * security context
References to abovementioned fields are changed to direct references
to matching fields of rte_ipsec_session structure.

Such refactoring is needed to introduce many sessions per SA feature,
e.g. fallback session for inline offload processing.

Signed-off-by: Marcin Smoczynski <marcinx.smoczynski@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>
Tested-by: Bernard Iremonger <bernard.iremonger@intel.com>