tipc: fix kernel warning when sending SYN message

In the Linux kernel, the following vulnerability has been resolved:

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 a41dad905e5a ("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.