netlink: mmap: fix tx type check

I can't send netlink message via mmaped netlink socket since

    commit: a8866ff6
    netlink: make the check for "send from tx_ring" deterministic

msg->msg_iter.type is set to WRITE (1) at

    SYSCALL_DEFINE6(sendto, ...
        import_single_range(WRITE, ...
            iov_iter_init(1, WRITE, ...

call path, so that we need to check the type by iter_is_iovec()
to accept the WRITE.
Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -2401,7 +2401,7 @@ static int netlink_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	 * sendmsg(), but that's what we've got...
 	 */
 	if (netlink_tx_is_mmaped(sk) &&
-	    msg->msg_iter.type == ITER_IOVEC &&
+	    iter_is_iovec(&msg->msg_iter) &&
 	    msg->msg_iter.nr_segs == 1 &&
 	    msg->msg_iter.iov->iov_base == NULL) {
 		err = netlink_mmap_sendmsg(sk, msg, dst_portid, dst_group,