ikev2: respect punting only for ipv4
ikev2: respect punting only for ipv4

IPSec punting to IKEv2 is valid only for NAT-T in IPv4.
Fix coverity CID 214915.

Type: fix

Change-Id: I6f2db38abf179565316f50c5d47c78acce3a0d01
Signed-off-by: Benoît Ganne <38aad8ffcfa407b890d258e568f161530f66882d@cisco.com>

chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp