block/backup: fix memory leak in bdrv_backup_top_append()

bdrv_open_driver() allocates bs->opaque according to drv->instance_size.
There is no need to allocate it and overwrite opaque in
bdrv_backup_top_append().

Reproducer:

  $ QTEST_QEMU_BINARY=./x86_64-softmmu/qemu-system-x86_64 valgrind -q --leak-check=full tests/test-replication -p /replication/secondary/start
  ==29792== 24 bytes in 1 blocks are definitely lost in loss record 52 of 226
  ==29792==    at 0x483AB1A: calloc (vg_replace_malloc.c:762)
  ==29792==    by 0x4B07CE0: g_malloc0 (in /usr/lib64/libglib-2.0.so.0.6000.7)
  ==29792==    by 0x12BAB9: bdrv_open_driver (block.c:1289)
  ==29792==    by 0x12BEA9: bdrv_new_open_driver (block.c:1359)
  ==29792==    by 0x1D15CB: bdrv_backup_top_append (backup-top.c:190)
  ==29792==    by 0x1CC11A: backup_job_create (backup.c:439)
  ==29792==    by 0x1CD542: replication_start (replication.c:544)
  ==29792==    by 0x1401B9: replication_start_all (replication.c:52)
  ==29792==    by 0x128B50: test_secondary_start (test-replication.c:427)
  ...

Fixes: 7df7868b ("block: introduce backup-top filter driver")
Signed-off-by: NEiichi Tsukata <devel@etsukata.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/backup-top.c

@@ -196,7 +196,7 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source,
     }
 
     top->total_sectors = source->total_sectors;
-    top->opaque = state = g_new0(BDRVBackupTopState, 1);
+    state = top->opaque;
 
     bdrv_ref(target);
     state->target = bdrv_attach_child(top, target, "target", &child_file, errp);