block: remove extra condition in bdrv_can_write_zeroes_with_unmap

All .bdrv_co_write_zeroes callbacks nowadays work perfectly even
with backing store attached. If future new callbacks would be unable to do
that - they have a chance to block this in bdrv_get_info().

Signed-off-by: Denis V. Lunev <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: John Snow <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Message-id: [email protected]
CC: Stefan Hajnoczi <[email protected]>
CC: Kevin Wolf <[email protected]>
CC: Max Reitz <[email protected]>
CC: Jeff Cody <[email protected]>
Signed-off-by: Jeff Cody <[email protected]>

Author: dlunev

block.c

@@ -2837,7 +2837,7 @@ bool bdrv_can_write_zeroes_with_unmap(BlockDriverState *bs)
 {
     BlockDriverInfo bdi;
 
-    if (bs->backing || !(bs->open_flags & BDRV_O_UNMAP)) {
+    if (!(bs->open_flags & BDRV_O_UNMAP)) {
         return false;
     }