selftests/net: Extract uring helpers to be reusable

Instead of defining basic io_uring functions in the test case, move them
to a common directory, so, other tests can use them.

This simplify the test code and reuse the common liburing
infrastructure. This is basically a copy of what we have in
io_uring_zerocopy_tx with some minor improvements to make checkpatch
happy.

A follow-up test will use the same helpers in a BPF sockopt test.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20231016134750.1381153-8-leitao@debian.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>