[SCSI] remove REQ_SPECIAL in scsi_init_io()

scsi_init_io() used to set REQ_SPECIAL when it fails sg
allocation before requeueing the request by returning
BLKPREP_DEFER.  REQ_SPECIAL is being updated to mean special
requests.  So, remove REQ_SPECIAL setting.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>