[SCSI] Redundant memset in scsi_alloc_sgtable

scsi_init_io calls scsi_alloc_sgtable and then calls blk_rq_map_sg
to initialize the scatterlist structure.  blk_rq_map_sg() already
memset the structure for every new segment.  That makes the memset
in scsi_alloc_sgtable unnecessary.

Patch to delete the extra memset in scsi_alloc_sgtable.  Tested on
a x86_64 machine.  Looks stable to me.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>