ipv4: introduce frag_expire_skip_icmp()

Improve readability of skip ICMP for de-fragmentation expiration logic.
This change will also make the logic easier to maintain when the
following patches in this series are applied.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/ip.h
Simple merge

diff --cc net/ipv4/ip_fragment.c
Simple merge