[NETLINK]: Add set/getsockopt options to support more than 32 groups

NETLINK_ADD_MEMBERSHIP/NETLINK_DROP_MEMBERSHIP are used to join/leave
groups, NETLINK_PKTINFO is used to enable nl_pktinfo control messages
for received packets to get the extended destination group number.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/socket.h b/include/linux/socket.h
index ddf2255..acc55aa 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -272,6 +272,7 @@
 #define SOL_NETBEUI	267
 #define SOL_LLC		268
 #define SOL_DCCP	269
+#define SOL_NETLINK	270
 
 /* IPX options */
 #define IPX_TYPE	1