io_uring/zcrx: add selftest

io_uring/zcrx: add selftest

Add a selftest for io_uring zero copy Rx. This test cannot run locally
and requires a remote host to be configured in net.config. The remote
host must have hardware support for zero copy Rx as listed in the
documentation page. The test will restore the NIC config back to before
the test and is idempotent.

liburing is required to compile the test and be installed on the remote
host running the test.

Signed-off-by: David Wei <dw@davidwei.uk>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20250215000947.789731-12-dw@davidwei.uk
Signed-off-by: Jens Axboe <axboe@kernel.dk>