sanlock: fix closing wrong client fd
    
    The symptoms of this bug were inq_lockspace returning
    ECONNRESET.  It was caused by a previous client closing
    the fd of a newer client doing inq_lockspace (when both
    clients were running at roughly the same time.)
    
    First client ci1, second client ci2.
    
    ci1 in call_cmd_daemon() is finished, and close(fd)
    is called (and client[ci].fd is *not* set to -1).
    
    ci2 is a new client at about the same time and gets the
    same fd that had been used by ci1.
    
    ci1 being finished triggers a poll error, which results
    in client_free(ci1).  client_free looks at client[ci1].fd
    and finds it is not -1, so it calls close() on it, but
    this fd is now being used by ci2.  This breaks the sanlock
    daemon connection for ci2 and the client gets ECONNRESET.