xen-blkback: Fix indentation

Avoid that smatch reports the following warning when building with
C=2 CHECK="smatch -p=kernel":

drivers/block/xen-blkback/blkback.c:710 xen_blkbk_unmap_prepare() warn: inconsistent indenting
Signed-off-by: NBart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: NHannes Reinecke <hare@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Roger Pau Monn303251 <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/xen-blkback/blkback.c

@@ -705,9 +705,9 @@ static unsigned int xen_blkbk_unmap_prepare(
 				    GNTMAP_host_map, pages[i]->handle);
 		pages[i]->handle = BLKBACK_INVALID_HANDLE;
 		invcount++;
-       }
+	}
 
-       return invcount;
+	return invcount;
 }
 
 static void xen_blkbk_unmap_and_respond_callback(int result, struct gntab_unmap_queue_data *data)