io_uring/net: support multishot for send

io_uring/net: support multishot for send

This works very much like the receive side, except for sends. The idea
is that an application can fill outgoing buffers in a provided buffer
group, and then arm a single send that will service them all. For now
this variant just terminates when we are out of buffers to send, and
hence the application needs to re-arm it if IORING_CQE_F_MORE isn't
set, as per usual for multishot requests.

This only enables it for IORING_OP_SEND, IORING_OP_SENDMSG is coming
in a separate patch. However, this patch does do a lot of the prep
work that makes wiring up the sendmsg variant pretty trivial. They
share the prep side.

Enabling multishot for sends is, again, identical to the receive side.
The app sets IORING_SEND_MULTISHOT in sqe->ioprio. This flag is also
the same as IORING_RECV_MULTISHOT.

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