[SCSI] libsas: use right function to alloc smp response

In fact the disc_resp buffer will be overwrite by smp response, so we never
found this typo, correct it by using the right one.

Signed-off-by: John Gong <john_gong@usish.com>
Signed-off-by: Jack Wang <jack_wang@usish.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>