block: Add PreallocMode to blk_truncate()

blk_truncate() itself will pass that value to bdrv_truncate(), and all
callers of blk_truncate() just set the parameter to PREALLOC_MODE_OFF
for now.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170613202107.10125-4-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/blockdev.c b/blockdev.c
index 124c230..9c6dd27 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2958,7 +2958,7 @@
     }
 
     bdrv_drained_begin(bs);
-    ret = blk_truncate(blk, size, errp);
+    ret = blk_truncate(blk, size, PREALLOC_MODE_OFF, errp);
     bdrv_drained_end(bs);
 
 out: