mmap: Invalidate initgroups memory cache after any change
    
    Initgroups memory cache was invalidated only in case on removed user.
    it should be invalidated also after changes in group membership.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2716
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>