ikev2: Destroy IKE_SA if INVALID_SYNTAX notify is received in response

ikev2: Destroy IKE_SA if INVALID_SYNTAX notify is received in response

RFC 7296, section 2.21.3:

   If a peer parsing a request notices that it is badly formatted (after
   it has passed the message authentication code checks and window
   checks) and it returns an INVALID_SYNTAX notification, then this
   error notification is considered fatal in both peers, meaning that
   the IKE SA is deleted without needing an explicit Delete payload.