VMCI: dma dg: allocate send and receive buffers for DMA datagrams

If DMA datagrams are used, allocate send and receive buffers
in coherent DMA memory.

This is done in preparation for the send and receive datagram
operations, where the buffers are used for the exchange of data
between driver and device.

Reviewed-by: Vishnu Dasa <vdasa@vmware.com>
Signed-off-by: Jorgen Hansen <jhansen@vmware.com>
Link: https://lore.kernel.org/r/20220207102725.2742-7-jhansen@vmware.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>