snmp: SNMP_UPD_PO_STATS_BH() always called from softirq

We dont need to test if we run from softirq context, we definitely are.

This saves few instructions in ip_rcv() & ip_rcv_finish()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>