sheepdog: fix dynamic grow for running qcow2 format

When running qcow2 over sheepdog, we might meet following problem

  qemu-system-x86_64: shrinking is not supported

And cause IO errors to Guest. This is because we abuse bs->total_sectors, which
is manipulated by generic block layer and race with sheepdog code.

We should directly check if offset > vdi_size to dynamically enlarge the volume
instead of 'offset > bs->total_sectors', which will cause problem when following
case happens:

   vdi_size > offset > bs->total_sectors

   # then trigger sd_truncate() to shrink the volume wrongly.

Cc: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: NHadrien KOHL <hadrien.kohl@gmail.com>
Signed-off-by: NLiu Yuan <namei.unix@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/sheepdog.c

@@ -2048,13 +2048,14 @@ static coroutine_fn int sd_co_writev(BlockDriverState *bs, int64_t sector_num,
 {
     SheepdogAIOCB *acb;
     int ret;
+    int64_t offset = (sector_num + nb_sectors) * BDRV_SECTOR_SIZE;
+    BDRVSheepdogState *s = bs->opaque;
 
-    if (bs->growable && sector_num + nb_sectors > bs->total_sectors) {
-        ret = sd_truncate(bs, (sector_num + nb_sectors) * BDRV_SECTOR_SIZE);
+    if (bs->growable && offset > s->inode.vdi_size) {
+        ret = sd_truncate(bs, offset);
         if (ret < 0) {
             return ret;
         }
-        bs->total_sectors = sector_num + nb_sectors;
     }
 
     acb = sd_aio_setup(bs, qiov, sector_num, nb_sectors);