io_uring/net: save msg_control for compat

Match the compat part of io_sendmsg_copy_hdr() with its counterpart and
save msg_control.

Fixes: c5597802 ("io_uring/net: move receive multishot out of the generic msghdr path")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2a8418821fe83d3b64350ad2b3c0303e9b732bbd.1740498502.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>