Do not print ICMP fields for non IP packets.

Do not print ICMP fields for non IP packets.

The packet's nw_proto field overlaps with the ARP opcode field.  The
nw_proto for ICMP packets is 1.  Hence when the packet is an ARP
request (opcode 1), we erroneously print the ICMP code and ICMP type
for ARP frames as well.  With this commit, the ICMP code and ICMP
type will be printed only if the packet is an IP packet and the
nw_proto is ICMP.

Signed-off-by: Mehak Mahajan <mmahajan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>