nf_nat: restrict ICMP translation for embedded header

 	Skip ICMP translation of embedded protocol header
if NAT bits are not set. Needed for IPVS to see the original
embedded addresses because for IPVS traffic the IPS_SRC_NAT_BIT
and IPS_DST_NAT_BIT bits are not set. It happens when IPVS performs
DNAT for client packets after using nf_conntrack_alter_reply
to expect replies from real server.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>