netdevsim: add ipsec offload testing

Implement the IPsec/XFRM offload API for testing.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>