btrfs: scrub: fix a return value overwrite in scrub_stripe()

[RETURN VALUE OVERWRITE]
Inside scrub_stripe(), we would submit all the remaining stripes after
iterating all extents.

But since flush_scrub_stripes() can return error, we need to avoid
overwriting the existing @ret if there is any error.

However the existing check is doing the wrong check:

	ret2 = flush_scrub_stripes();
	if (!ret2)
		ret = ret2;

This would overwrite the existing @ret to 0 as long as the final flush
detects no critical errors.

[FIX]
We should check @ret other than @ret2 in that case.

Fixes: 8eb3dd17 ("btrfs: dev-replace: error out if we have unrepaired metadata error during")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -2266,7 +2266,7 @@ static noinline_for_stack int scrub_stripe(struct scrub_ctx *sctx,
 	}
 out:
 	ret2 = flush_scrub_stripes(sctx);
-	if (!ret2)
+	if (!ret)
 		ret = ret2;
 	if (sctx->raid56_data_stripes) {
 		for (int i = 0; i < nr_data_stripes(map); i++)