dlm_controld: set SO_RCVBUF for netlink socket
    
    Saw some:
    
    1597148652 uevent recv error -1 errno 105
    
    on a dlm_tool dump. The errno 105 is ENOBUFS on an recv of an AF_NETLINK
    socket. Further investigations showed that we dropping uevents in such
    case, see the added comment. The above error message was on a node which
    hung inside do_uevent() of dlm kernel code which means that the
    node is waiting for a sysfs write of "event_done". My guess is that
    dlm_controld dropped some "important" messages and never writes to
    "event_done" in this case. However we should prevent such ENOBUFS cases
    in netlink which this patch is trying to do in a simple way.