net: Avoid access icmp_err_convert when icmp code is ICMP_FRAG_NEEDED

There is no need to fetch errno and fatal info from icmp_err_convert when
icmp code is ICMP_FRAG_NEEDED.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>