NSS: remove timed event if related object is removed
    
    setnetgrent_result_timeout() is called as a timed event to free the
    netgr data is the cache lifetime is exceeded. If the data is freed
    earlier the timed event should be removed as well to avoid a double
    free issue.
    
    Since talloc is used here the most easy way to achieve this is to
    allocate the timed event on the netgr object itself.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3737
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>