scsi: libsas: fix length error in sas_smp_handler()

The return value of smp_execute_task_sg() is the untransferred residual,
but bsg_job_done() requires the length of payload received. This makes
SMP passthrough commands from userland by sg ioctl to libsas get a wrong
response. The userland tools such as smp_utils failed because of these
wrong responses:

~#smp_discover /dev/bsg/expander-2\:13
response too short, len=0
~#smp_discover /dev/bsg/expander-2\:134
response too short, len=0

Fix this by passing the actual received length to bsg_job_done(). And if
smp_execute_task_sg() returns 0, this means received length is exactly
the buffer length.

[mkp: typo]

Fixes: 651a013 ("scsi: scsi_transport_sas: switch to bsg-lib for SMP passthrough")
Cc: <stable@vger.kernel.org> # v4.14+
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reported-by: chenqilin <chenqilin2@huawei.com>
Tested-by: chenqilin <chenqilin2@huawei.com>
CC: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libsas/sas_expander.c

@@ -2145,7 +2145,7 @@ void sas_smp_handler(struct bsg_job *job, struct Scsi_Host *shost,
 		struct sas_rphy *rphy)
 {
 	struct domain_device *dev;
-	unsigned int reslen = 0;
+	unsigned int rcvlen = 0;
 	int ret = -EINVAL;
 
 	/* no rphy means no smp target support (ie aic94xx host) */
@@ -2179,12 +2179,12 @@ void sas_smp_handler(struct bsg_job *job, struct Scsi_Host *shost,
 
 	ret = smp_execute_task_sg(dev, job->request_payload.sg_list,
 			job->reply_payload.sg_list);
-	if (ret > 0) {
-		/* positive number is the untransferred residual */
-		reslen = ret;
+	if (ret >= 0) {
+		/* bsg_job_done() requires the length received  */
+		rcvlen = job->reply_payload.payload_len - ret;
 		ret = 0;
 	}
 
 out:
-	bsg_job_done(job, ret, reslen);
+	bsg_job_done(job, ret, rcvlen);
 }