selftests: ncdevmem: Implement devmem TCP TX

Add support for devmem TX in ncdevmem.

This is a combination of the ncdevmem from the devmem TCP series RFCv1
which included the TX path, and work by Stan to include the netlink API
and refactored on top of his generic memory_provider support.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250508004830.4100853-10-almasrymina@google.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>