net: filter: move forward declarations to avoid compile warnings

Get rid of this compile warning:

In file included from arch/s390/kernel/compat_linux.c:37:0:
include/linux/filter.h:139:23: warning: 'struct sk_buff' declared inside parameter list
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/filter.h

@@ -131,6 +131,10 @@ struct sock_fprog {	/* Required for SO_ATTACH_FILTER. */
 #define SKF_LL_OFF    (-0x200000)
 
 #ifdef __KERNEL__
+
+struct sk_buff;
+struct sock;
+
 struct sk_filter
 {
 	atomic_t		refcnt;
@@ -146,9 +150,6 @@ static inline unsigned int sk_filter_len(const struct sk_filter *fp)
 	return fp->len * sizeof(struct sock_filter) + sizeof(*fp);
 }
 
-struct sk_buff;
-struct sock;
-
 extern int sk_filter(struct sock *sk, struct sk_buff *skb);
 extern unsigned int sk_run_filter(const struct sk_buff *skb,
 				  const struct sock_filter *filter);