[SCSI] sd: Permit merged discard requests

Support requests with more than one bio payload for discards. The total
number of bytes to be discarded is stored in req->__data_len and used in
sd_done() to complete the I/O.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>