net: Make userland include of netlink.h more sane.

Currently userland will barf when including linux/netlink.h unless it
precisely includes sys/socket.h first.  The issue is where the
definition of "sa_family_t" comes from.

We've been back and forth on how to fix this issue in the past, see:

http://thread.gmane.org/gmane.linux.debian.devel.bugs.general/622621
http://thread.gmane.org/gmane.linux.network/143380

Ben Hutchings suggested we take a hint from how we handle the
sockaddr_storage type.  First we define a "__kernel_sa_family_t"
to linux/socket.h that is always defined.

Then if __KERNEL__ is defined, we also define "sa_family_t" as
equal to "__kernel_sa_family_t".

Then in places like linux/netlink.h we use __kernel_sa_family_t
in user visible datastructures.

Reported-by: Michel Machado <michel@digirati.com.br>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -29,7 +29,7 @@
 #define MAX_LINKS 32		
 
 struct sockaddr_nl {
-	sa_family_t	nl_family;	/* AF_NETLINK	*/
+	__kernel_sa_family_t	nl_family;	/* AF_NETLINK	*/
 	unsigned short	nl_pad;		/* zero		*/
 	__u32		nl_pid;		/* port ID	*/
        	__u32		nl_groups;	/* multicast groups mask */