block: Fix dirty bitmap in bdrv_co_discard

Unsetting dirty globally with discard is not very correct. The discard may zero
out sectors (depending on can_write_zeroes_with_unmap), we should replicate
this change to destination side to make sure that the guest sees the same data.

Calling bdrv_reset_dirty also troubles mirror job because the hbitmap iterator
doesn't expect unsetting of bits after current position.

So let's do it the opposite way which fixes both problems: set the dirty bits
if we are to discard it.

Reported-by: wangxiaolong@ucloud.cn
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 50824995)
Conflicts:
	block/io.c

* applied manually to avoid dependency on 61007b31
* squashed in 6e82e4bc bdrv_reset_dirty() is static in
  2.3.0 and becomes unused as of this patch
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

block.c

@@ -99,8 +99,6 @@ static QLIST_HEAD(, BlockDriver) bdrv_drivers =
 
 static void bdrv_set_dirty(BlockDriverState *bs, int64_t cur_sector,
                            int nr_sectors);
-static void bdrv_reset_dirty(BlockDriverState *bs, int64_t cur_sector,
-                             int nr_sectors);
 /* If non-zero, use only whitelisted block drivers */
 static int use_bdrv_whitelist;
 
@@ -5299,8 +5297,6 @@ int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
         return -EROFS;
     }
 
-    bdrv_reset_dirty(bs, sector_num, nb_sectors);
-
     /* Do nothing if disabled.  */
     if (!(bs->open_flags & BDRV_O_UNMAP)) {
         return 0;
@@ -5310,6 +5306,8 @@ int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
         return 0;
     }
 
+    bdrv_set_dirty(bs, sector_num, nb_sectors);
+
     max_discard = MIN_NON_ZERO(bs->bl.max_discard, BDRV_REQUEST_MAX_SECTORS);
     while (nb_sectors > 0) {
         int ret;
@@ -5620,15 +5618,6 @@ static void bdrv_set_dirty(BlockDriverState *bs, int64_t cur_sector,
     }
 }
 
-static void bdrv_reset_dirty(BlockDriverState *bs, int64_t cur_sector,
-                             int nr_sectors)
-{
-    BdrvDirtyBitmap *bitmap;
-    QLIST_FOREACH(bitmap, &bs->dirty_bitmaps, list) {
-        hbitmap_reset(bitmap->bitmap, cur_sector, nr_sectors);
-    }
-}
-
 int64_t bdrv_get_dirty_count(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
 {
     return hbitmap_count(bitmap->bitmap);