Allow userspace to submit synchronous I/O like the SCSI sg interface does.
Signed-off-by: NMatthew Wilcox <matthew.r.wilcox@intel.com>

Add prp1, prp2 and the metadata prp to the common command, since the
fields are generally used this way.
Signed-off-by: NMatthew Wilcox <matthew.r.wilcox@intel.com>

This driver is for devices that follow the NVM Express standard
Signed-off-by: NMatthew Wilcox <matthew.r.wilcox@intel.com>