io_uring/cmd: move net cmd into a separate file

We keep socket io_uring command implementation in io_uring/uring_cmd.c.
Separate it from generic command code into a separate file.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/747d0519a2255bd055ae76b691d38d2b4c311001.1745843119.git.asml.silence@gmail.com


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