xen-blkback: prefer xenbus_scanf() over xenbus_gather()

... for single items being collected: It is more typesafe (as the
compiler can check format string and to-be-written-to variable match)
and requires one less parameter to be passed.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkback/xenbus.c

@@ -1022,9 +1022,9 @@ static int connect_ring(struct backend_info *be)
 	pr_debug("%s %s\n", __func__, dev->otherend);
 
 	be->blkif->blk_protocol = BLKIF_PROTOCOL_DEFAULT;
-	err = xenbus_gather(XBT_NIL, dev->otherend, "protocol",
-			    "%63s", protocol, NULL);
-	if (err)
+	err = xenbus_scanf(XBT_NIL, dev->otherend, "protocol",
+			   "%63s", protocol);
+	if (err <= 0)
 		strcpy(protocol, "unspecified, assuming default");
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_NATIVE))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_NATIVE;
@@ -1036,10 +1036,9 @@ static int connect_ring(struct backend_info *be)
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		return -ENOSYS;
 	}
-	err = xenbus_gather(XBT_NIL, dev->otherend,
-			    "feature-persistent", "%u",
-			    &pers_grants, NULL);
-	if (err)
+	err = xenbus_scanf(XBT_NIL, dev->otherend,
+			   "feature-persistent", "%u", &pers_grants);
+	if (err <= 0)
 		pers_grants = 0;
 
 	be->blkif->vbd.feature_gnt_persistent = pers_grants;