io_uring/net: use io_sr_msg for sendzc

Reuse struct io_sr_msg for zerocopy sends, which is handy. There is
only one zerocopy specific field, namely .notif, and we have enough
space for it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/408c5b1b2d8869e1a12da5f5a78ed72cac112149.1662639236.git.asml.silence@gmail.com


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