io_uring/net: use unsigned for flags

Use unsigned int type for msg flags.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5cfaed13d3191337b14b8664ca68b515d9e2d1b4.1658742118.git.asml.silence@gmail.com


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