NVMe: Add scsi unmap to SG_IO

Translates a scsi unmap request from SG_IO ioctl to NVMe
data-set-management deallocate.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Acked-by: Vishal Verma <vishal.l.verma@linux.intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>