sanlock: use io_destroy to clear outstanding aio
    
    When a lockspace thread attempts to clear outstanding
    aio ops, it was using io_getevents indefinitely to wait
    for completions.  Now, it uses io_getevents to clear the
    ops for up to 2 minutes, after which it uses io_destroy
    to clear them.  io_destroy may have the ability clear
    certain ops that will never have a completion.