illumos gate

#14273 documented TX and RX ring support by mac(9E). However it neglects to mention that the m_v12n field of the mac_register_t instance that is passed to mac_register(9F) must be set to MAC_VIRT_LEVEL1 before the mac layer will even attempt to check if the driver supports MAC_CAPAB_RINGS.

As a possible additional bug, passing a mac_register_t instance that conforms to all of the requirements of MAC_CAPAB_RINGS (e.g. the m_callbacks.mc_{tx,unicst} methods are NULL) except for the missing m_v12n value will successfully register, however on the first attempt to tx, the system will panic because mac_provider_tx is called with a NULL mac_ring_handle_t.