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>

fs/btrfs/scrub.c

@@ -2100,7 +2100,7 @@ static int scrub_simple_mirror(struct scrub_ctx *sctx,
 	struct btrfs_fs_info *fs_info = sctx->fs_info;
 	const u64 logical_end = logical_start + logical_length;
 	u64 cur_logical = logical_start;
-	int ret;
+	int ret = 0;
 
 	/* The range must be inside the bg */
 	ASSERT(logical_start >= bg->start && logical_end <= bg->start + bg->length);