net: snmp: fix the wrong ICMP_MIB_MAX value

__ICMP_MIB_MAX is equal to the total number of icmp mib,
So no need to add 1. This wastes 4/8 bytes memory.

Change it to be same as ICMP6_MIB_MAX, TCP_MIB_MAX, UDP_MIB_MAX.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>