socket: Avoid possible race between netmsg_so_notify() and sowakeup()

The race is concerning SSB_MEVENT setup and testing.  Originally:

             CPU X                          CPU Y
    get token                                 :
    if (predict cond false) {                 :
           :                        set predict cond true
           :                        if (SSB_MEVENT) {
           :                            get token
           :                            wake up notifies
           :                            rel token
           :                        }
        queue notify                          :
        set SSB_MEVENT                        :
    }                                         :
    rel token

This commit always set SSB_MEVENT before predict testing, after getting
the socket token, so the race could be avoided.

Reviewed-by: dillon@