Fix several callout migration races:  - Problem1:    Hypothesis: thread1 is doing a callout_reset_on(), within his    callout handler, willing to implicitly or explicitly migrate the    callout.  thread2 is draining the callout.
Fix several callout migration races:
 - Problem1:
   Hypothesis: thread1 is doing a callout_reset_on(), within his
   callout handler, willing to implicitly or explicitly migrate the
   callout.  thread2 is draining the callout.

   Thesys:
   * thread1 calls callout_lock() and locks the old callout cpu
   * thread1 performs the checks in the first path of the
     callout_reset_on()
   * thread1 hits this codepiece:
       /*
        * If the lock must migrate we have to check the state again as
        * we can't hold both the new and old locks simultaneously.
        */
       if (c->c_cpu != cpu) {
               c->c_cpu = cpu;
               CC_UNLOCK(cc);
               goto retry;
       }

     which means it will drop the lock and 'retry'
   * thread2 will callout_lock() and locks the new callout cpu.
     thread1 spins on the new lock and will not keep going for the
     moment.
   * thread2 checks that the callout is not pending (as callout is
     currently running) and that it is not on cc->cc_curr (because cc
     now refers to the new callout and the callout is running on the
     old callout cpu) thus it thinks it is done and returns.
   * thread1  will now acquire the lock and then adds the callout
     to the new callout cpu queue

   That seems an obvious race as callout_stop() falsely reports
   the callout stopped or worse, callout_drain() falsely returns
   while the callout is still in use.
 - Solution1:
   Fixing this problem would require, in general, to lock both
   callout cpus at once while switching the c_cpu field and avoid
   cyclic deadlocks between callout cpus locks.
   The concept of CPUBLOCK is then introduced (working more or less
   like the blocked_lock for thread_lock() function) meaning:
   "in callout_lock(), spin until the c->c_cpu is not different from
   CPUBLOCK". That way the "original" callout cpu, referred to the
   above mentioned code snippet, will remain blocked until the lock
   handover is over critical path will remain covered.

 - Problem2:
   Having the callout currently executed on a specific callout cpu
   and contemporary pending on another callout cpu (as it can happen
   with current code) breaks, at least, the assumption callout_drain()
   returns just once the callout cannot be referenced anymore.
 - Solution2:
   Callout migration is deferred if the current callout is already
   under execution.
   The best place to do that is in softclock() and new members are
   added to the callout cpu structure in order to specify a pending
   migration is requested. That is necessary because the callout
   cannot be trusted (not freed) the 100% of times after the execution
   of the callout handler.
   CPUBLOCK will prevent, in the "deferred migration" case, that the
   callout gets freed in this case, stopping any callout_stop() and
   callout_drain() possible activity until the migration is
   actually performed.

 - Problem3:
   There is a further race in callout_drain().
   In order to avoid a race between sleepqueue lock and callout cpu
   spinlock, in _callout_stop_safe(), the callout cpu lock is dropped,
   the sleepqueue lock is acquired and a new callout cpu lookup is
   performed.  Note that the channel used for locking the sleepqueue is
   obtained from the "current" callout cpu (&cc->cc_waiting).
   If the callout migrated in the meanwhile, callout_drain() will end up
   using the wrong wchan for the sleepqueue (the locked one will be the
   older, while the new one will not really be locked) leading to a
   lock leak and a race access to sleepqueue.
 - Solution3:
   It is enough to check if a migration happened between the operation
   of acquiring the sleepqueue lock and the new callout cpu lock and
   eventually unwind all those and try again.

This problems can lead to deathly races on moderate (4-ways) SMP
environment, leading to easy panic or deadlocks.
The 24-ways of the reporter, could easilly panic, with completely
normal workload, almost daily.
gianni@ kindly wrote the following prof-of-concept which can
panic a FreeBSD machine in less than one hour, in smaller SMP:
http://www.freebsd.org/~attilio/callout/test.c

Reported by:	Nicholas Esborn <nick at desert dot net>, DesertNet
In collabouration with:	gianni, pho, Nicholas Esborn
Reviewed by:	jhb
MFC after:	1 week (*)

* Usually, I would aim for a larger MFC timeout, but I really want this
  in before 8.2-RELEASE, thus re@ accepted a shorter timeout as a special
  case for this patch

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