nwfilter: use standard label names when reasonable

nwfilter: use standard label names when reasonable

Rather than having labels named exit, done, exit_snooprequnlock,
skip_rename, etc, use the standard "cleanup" label. And instead of
err_exit, malformed, tear_down_tmpebchains, use "error".

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>