btrfs: scrub: initialize ret in scrub_simple_mirror() to fix compilation warning

The following error message is displayed:
  ../fs/btrfs/scrub.c:2152:9: error: ‘ret’ may be used uninitialized
  in this function [-Werror=maybe-uninitialized]"

Compiler version: gcc version: (Debian 10.2.1-6) 10.2.1 20210110

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Lu Yao <yaolu@kylinos.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>