io_uring/net: use the correct msghdr union member in io_sendmsg_copy_hdr

Rather than assign the user pointer to msghdr->msg_control, assign it
to msghdr->msg_control_user to make sparse happy. They are in a union
so the end result is the same, but let's avoid new sparse warnings and
squash this one.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306210654.mDMcyMuB-lkp@intel.com/
Fixes: cac9e441

 ("io_uring/net: save msghdr->msg_control for retries")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>