[NETLINK]: Use group numbers instead of bitmasks internally

Using the group number allows increasing the number of groups without
beeing limited by the size of the bitmask. It introduces one limitation
for netlink users: messages can't be broadcasted to multiple groups anymore,
however this feature was never used inside the kernel.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index eab51f9..c724c9d 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -107,7 +107,7 @@
 	struct ucred		creds;		/* Skb credentials	*/
 	__u32			pid;
 	__u32			dst_pid;
-	__u32			dst_groups;
+	__u32			dst_group;
 	kernel_cap_t		eff_cap;
 	__u32			loginuid;	/* Login (audit) uid */
 };