block: Use bdrv_nb_sectors() in bdrv_make_zero()

Instead of bdrv_getlength().

Variable target_size is initially in bytes, then changes meaning to
sectors.  Ugh.  Replace by target_sectors.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -2828,18 +2828,16 @@ int bdrv_write_zeroes(BlockDriverState *bs, int64_t sector_num,
  */
 int bdrv_make_zero(BlockDriverState *bs, BdrvRequestFlags flags)
 {
-    int64_t target_size;
-    int64_t ret, nb_sectors, sector_num = 0;
+    int64_t target_sectors, ret, nb_sectors, sector_num = 0;
     int n;
 
-    target_size = bdrv_getlength(bs);
-    if (target_size < 0) {
-        return target_size;
+    target_sectors = bdrv_nb_sectors(bs);
+    if (target_sectors < 0) {
+        return target_sectors;
     }
-    target_size /= BDRV_SECTOR_SIZE;
 
     for (;;) {
-        nb_sectors = target_size - sector_num;
+        nb_sectors = target_sectors - sector_num;
         if (nb_sectors <= 0) {
             return 0;
         }