netlink: fix uninit-value in netlink_sendmsg

syzbot reported :

BUG: KMSAN: uninit-value in ffs arch/x86/include/asm/bitops.h:432 [inline]
BUG: KMSAN: uninit-value in netlink_sendmsg+0xb26/0x1310 net/netlink/af_netlink.c:1851

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -1844,6 +1844,8 @@ static int netlink_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 
 	if (msg->msg_namelen) {
 		err = -EINVAL;
+		if (msg->msg_namelen < sizeof(struct sockaddr_nl))
+			goto out;
 		if (addr->nl_family != AF_NETLINK)
 			goto out;
 		dst_portid = addr->nl_pid;