NTB: MFV c9d534c8: Correctly handle receive buffers of the minimal size

The ring logic of the NTB receive buffer/transmit memory window requires
there to be at least 2 payload sized allotments. For the minimal size
case, split the buffer into two and set the transport_mtu to the
appropriate size.

Authored by: Jon Mason
Obtained from: Linux
Sponsored by: EMC / Isilon Storage Division