scsi: fix random memory corruption with scsi-mq + T10 PI

This fixes random memory corruption triggered when all three of the
following are true:

* scsi-mq enabled
* T10 Protection Information (DIF) enabled
* SCSI host with sg_tablesize > SCSI_MAX_SG_SEGMENTS (128)

The symptoms of this bug are unpredictable memory corruption, BUG()s,
oopses, lockups, etc., any of which may appear to be completely
unrelated to the root cause.

Cc: <stable@vger.kernel.org> # 3.17.x, 3.18.x
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Reviewed-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/scsi_lib.c

@@ -1918,7 +1918,9 @@ static int scsi_mq_prep_fn(struct request *req)
 
 	if (scsi_host_get_prot(shost)) {
 		cmd->prot_sdb = (void *)sg +
-			shost->sg_tablesize * sizeof(struct scatterlist);
+			min_t(unsigned int,
+			      shost->sg_tablesize, SCSI_MAX_SG_SEGMENTS) *
+			sizeof(struct scatterlist);
 		memset(cmd->prot_sdb, 0, sizeof(struct scsi_data_buffer));
 
 		cmd->prot_sdb->table.sgl =