[SCSI] mpt2sas: fix the incorrect scsi_dma_map error checking

scsi_dma_map() returns -1 if an error occurred (zero means that the
command has no data). So the following current code can't catch an
error:

sges_left = scsi_dma_map(scmd);
if (!sges_left) {
	sdev_printk(KERN_ERR, scmd->device, "pci_map_sg"
	" failed: request for %d bytes!\n", scsi_bufflen(scmd));
	return -ENOMEM;
}

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: "Desai, Kashyap" <Kashyap.Desai@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

drivers/scsi/mpt2sas/mpt2sas_scsih.c

@@ -988,7 +988,7 @@ _scsih_build_scatter_gather(struct MPT2SAS_ADAPTER *ioc,
 	u32 chain_offset;
 	u32 chain_length;
 	u32 chain_flags;
-	u32 sges_left;
+	int sges_left;
 	u32 sges_in_segment;
 	u32 sgl_flags;
 	u32 sgl_flags_last_element;
@@ -1009,7 +1009,7 @@ _scsih_build_scatter_gather(struct MPT2SAS_ADAPTER *ioc,
 
 	sg_scmd = scsi_sglist(scmd);
 	sges_left = scsi_dma_map(scmd);
-	if (!sges_left) {
+	if (sges_left < 0) {
 		sdev_printk(KERN_ERR, scmd->device, "pci_map_sg"
 		" failed: request for %d bytes!\n", scsi_bufflen(scmd));
 		return -ENOMEM;