selftests: net: add devmem RX and TX netdev unregister tests

Add devmem_bind_rx_unregister_check and devmem_bind_tx_unregister_check
to nl_netdev.py to verify that unregistering a netdevsim device while a
netlink socket holds an active RX or TX devmem binding synchronously
detaches the dma_buf attachment and cleanly closes the netlink socket
without use-after-free or page faults.

Add the dma-buf, udmabuf and devmem options the tests need to
tools/testing/selftests/net/config.

Cc: Tariq Toukan <tariqt@nvidia.com>
Cc: Kaifeng Wang <kaifengw@google.com>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20261030083539.777877-5-almasrymina@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>