xen/blkback: Align the tabs on the structure.

The recent changes caused this field of the structure to be offset a bit.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/xenbus.c

@@ -23,7 +23,7 @@
 
 struct backend_info {
 	struct xenbus_device	*dev;
-	struct xen_blkif		*blkif;
+	struct xen_blkif	*blkif;
 	struct xenbus_watch	backend_watch;
 	unsigned		major;
 	unsigned		minor;