bpf: make function xdp_do_generic_redirect_map() static

The function xdp_do_generic_redirect_map() is only used in this file, so
make it static.

Clean up sparse warning:
net/core/filter.c:2687:5: warning: no previous prototype
for 'xdp_do_generic_redirect_map' [-Wmissing-prototypes]
Signed-off-by: NXiongwei Song <sxwjean@gmail.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

net/core/filter.c

@@ -2684,7 +2684,8 @@ static int __xdp_generic_ok_fwd_dev(struct sk_buff *skb, struct net_device *fwd)
 	return 0;
 }
 
-int xdp_do_generic_redirect_map(struct net_device *dev, struct sk_buff *skb,
+static int xdp_do_generic_redirect_map(struct net_device *dev,
+				       struct sk_buff *skb,
 				       struct bpf_prog *xdp_prog)
 {
 	struct redirect_info *ri = this_cpu_ptr(&redirect_info);