Allocate and use a DMA engine channel to transmit and receive data over
NTB.  If none is allocated, fall back to using the CPU to transfer data.
Signed-off-by: NJon Mason <jon.mason@intel.com>
Reviewed-by: NDan Williams <dan.j.williams@intel.com>
Reviewed-by: NDave Jiang <dave.jiang@intel.com>