ipvs: ensure that ICMP cannot be sent in reply to ICMP

Check the header for icmp before sending a PACKET_TOO_BIG

Signed-off-by: Alex Gartrell <agartrell@fb.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>