scsi: ufs-bsg: Allow reading descriptors

Add this functionality, placing the descriptor being read in the actual
data buffer in the bio.

That is, for both read and write descriptors query upiu, we are using the
job's request_payload.  This in turn, is mapped back in user land to the
applicable sg_io_v4 xferp: dout_xferp for write descriptor, and din_xferp
for read descriptor.

Signed-off-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Evan Green <evgreen@chromium.org>
Reviewed-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>