NTB: 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.

Signed-off-by: Jon Mason <jon.mason@intel.com>

diff --cc drivers/ntb/ntb_transport.c
Simple merge