io_uring: separate header for exported net bits

We're exporting some io_uring bits to networking, e.g. for implementing
a net callback for io_uring cmds, but we don't want to expose more than
needed. Add a separate header for networking.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: David Wei <dw@davidwei.uk>
Link: https://lore.kernel.org/r/20240409210554.1878789-1-dw@davidwei.uk


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