MFC 222032:

MFC 222032:
Fix a race in the SMP rendezvous code.  Specifically, the write by the
last CPU to to finish the rendezvous action may become visible to
different CPUs at different times.  As a result, the CPU that initiated
the rendezvous may exit the rendezvous and drop the lock allowing another
rendezvous to be initiated on the same CPU or a different CPU.  In that
case the exit sentinel may be cleared before all CPUs have noticed causing
those CPUs to hang forever.

Workaround this by using a generation count to notice when this race
occurs and to exit the rendezvous in that case.

git-svn-id: svn://svn.freebsd.org/base/stable/8@222938 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f