net: enable driver support for netmem TX

Drivers need to make sure not to pass netmem dma-addrs to the
dma-mapping API in order to support netmem TX.

Add helpers and netmem_dma_*() helpers that enables special handling of
netmem dma-addrs that drivers can use.

Document in netmem.rst what drivers need to do to support netmem TX.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250508004830.4100853-7-almasrymina@google.com


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