fs-util,tmpfiles: fix error handling of fchmod_opath()

When 4dfaa528 was first commited its callers relied on `errno` instead of the
return value for error reporting. Which worked fine, since internally
under all conditions base were set — even if ugly and not inline with
our coding style. Things then got broken in
f8606626 where suddenly additional
syscalls might end up being done in the function, thus corrupting `errno`.