selftests: add ncdevmem, netcat for devmem TCP

selftests: add ncdevmem, netcat for devmem TCP

ncdevmem is a devmem TCP netcat. It works similarly to netcat, but it
sends and receives data using the devmem TCP APIs. It uses udmabuf as
the dmabuf provider. It is compatible with a regular netcat running on
a peer, or a ncdevmem running on a peer.

In addition to normal netcat support, ncdevmem has a validation mode,
where it sends a specific pattern and validates this pattern on the
receiver side to ensure data integrity.

Suggested-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20240910171458.219195-13-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>