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>

net/ipv4/bpfilter/sockopt.c

@@ -10,7 +10,8 @@ int (*bpfilter_process_sockopt)(struct sock *sk, int optname,
 				unsigned int optlen, bool is_set);
 EXPORT_SYMBOL_GPL(bpfilter_process_sockopt);
 
-int bpfilter_mbox_request(struct sock *sk, int optname, char __user *optval,
+static int bpfilter_mbox_request(struct sock *sk, int optname,
+				 char __user *optval,
 				 unsigned int optlen, bool is_set)
 {
 	if (!bpfilter_process_sockopt) {