missing_error_code2: warn about return ret where ret is zero

missing_error_code2: warn about return ret where ret is zero

This is for code like:

	ret = frob();
	if (ret)
		return ret;

	if (len != 10)
		return ret;

Sometimes (quite often maybe?) the code really is supposed to return 0 but
often it's supposed to return an error code.  Even if zero is intended it's
more readable to return a literal zero instead of "ret".

This will not warn about code like:

	ret = frob();
	if (!ret)
		return ret;

I have an unpublished check for code like that.  Those are normally
intentional but the canonical bug for that type of code is a reversed if
statement where the "!" was unintentional.  I feel like "return 0;" is
much better code in these cases as well, but I don't like getting into
draining style debates when there are so many real bugs which I don't
have time to fix.

Signed-off-by: Dan Carpenter <error27@gmail.com>