net: bpfilter: make function bpfilter_mbox_request() static

Fixes the following sparse warnings:

net/ipv4/bpfilter/sockopt.c:13:5: warning:
 symbol 'bpfilter_mbox_request' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>