net: Move "struct net" declaration inside the __KERNEL__ macro guard

This patch reduces namespace pollution by moving the "struct net" declaration
out of the userspace-facing portion of linux/netlink.h.  It has no impact on
the kernel.

(This came up because we have several C++ applications which use "net" as a
namespace name.)
Signed-off-by: Ollie Wild <aaw@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -27,8 +27,6 @@
 
 #define MAX_LINKS 32		
 
-struct net;
-
 struct sockaddr_nl {
 	sa_family_t	nl_family;	/* AF_NETLINK	*/
 	unsigned short	nl_pad;		/* zero		*/
@@ -151,6 +149,8 @@ struct nlattr {
 #include <linux/capability.h>
 #include <linux/skbuff.h>
 
+struct net;
+
 static inline struct nlmsghdr *nlmsg_hdr(const struct sk_buff *skb)
 {
 	return (struct nlmsghdr *)skb->data;