examples/ipsec-secgw: support fallback session

examples/ipsec-secgw: support fallback session

Inline processing is limited to a specified subset of traffic. It is
often unable to handle more complicated situations, such as fragmented
traffic. When using inline processing such traffic is dropped.

Introduce fallback session for inline crypto processing allowing
handling packets that normally would be dropped. A fallback session is
configured by adding 'fallback' keyword with 'lookaside-none' parameter
to an SA configuration. Only 'inline-crypto-offload" as a primary
session and 'lookaside-none' as a fall-back session combination is
supported by this patch.

Fallback session feature is not available in the legacy mode.

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