tipc: fix kernel warning when sending SYN message

When sending a SYN message, this kernel stack trace is observed:

...
[   13.396352] RIP: 0010:_copy_from_iter+0xb4/0x550
...
[   13.398494] Call Trace:
[   13.398630]  <TASK>
[   13.398630]  ? __alloc_skb+0xed/0x1a0
[   13.398630]  tipc_msg_build+0x12c/0x670 [tipc]
[   13.398630]  ? shmem_add_to_page_cache.isra.71+0x151/0x290
[   13.398630]  __tipc_sendmsg+0x2d1/0x710 [tipc]
[   13.398630]  ? tipc_connect+0x1d9/0x230 [tipc]
[   13.398630]  ? __local_bh_enable_ip+0x37/0x80
[   13.398630]  tipc_connect+0x1d9/0x230 [tipc]
[   13.398630]  ? __sys_connect+0x9f/0xd0
[   13.398630]  __sys_connect+0x9f/0xd0
[   13.398630]  ? preempt_count_add+0x4d/0xa0
[   13.398630]  ? fpregs_assert_state_consistent+0x22/0x50
[   13.398630]  __x64_sys_connect+0x16/0x20
[   13.398630]  do_syscall_64+0x42/0x90
[   13.398630]  entry_SYSCALL_64_after_hwframe+0x63/0xcd

It is because commit a41dad90 ("iov_iter: saner checks for attempt
to copy to/from iterator") has introduced sanity check for copying
from/to iov iterator. Lacking of copy direction from the iterator
viewpoint would lead to kernel stack trace like above.

This commit fixes this issue by initializing the iov iterator with
the correct copy direction when sending SYN or ACK without data.

Fixes: f25dcc76 ("tipc: tipc ->sendmsg() conversion")
Reported-by: syzbot+d43608d061e8847ec9f3@syzkaller.appspotmail.com
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Link: https://lore.kernel.org/r/20230214012606.5804-1-tung.q.nguyen@dektech.com.auSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/tipc/socket.c

@@ -2614,6 +2614,7 @@ static int tipc_connect(struct socket *sock, struct sockaddr *dest,
 		/* Send a 'SYN-' to destination */
 		m.msg_name = dest;
 		m.msg_namelen = destlen;
+		iov_iter_kvec(&m.msg_iter, ITER_SOURCE, NULL, 0, 0);
 
 		/* If connect is in non-blocking case, set MSG_DONTWAIT to
 		 * indicate send_msg() is never blocked.
@@ -2776,6 +2777,7 @@ static int tipc_accept(struct socket *sock, struct socket *new_sock, int flags,
 		__skb_queue_head(&new_sk->sk_receive_queue, buf);
 		skb_set_owner_r(buf, new_sk);
 	}
+	iov_iter_kvec(&m.msg_iter, ITER_SOURCE, NULL, 0, 0);
 	__tipc_sendstream(new_sock, &m, 0);
 	release_sock(new_sk);
 exit: