rcuwait: Introduce prepare_to and finish_rcuwait

This allows further flexibility for some callers to implement
ad-hoc versions of the generic rcuwait_wait_event(). For example,
kvm will need this to maintain tracing semantics. The naming
is of course similar to what waitqueue apis offer.

Also go ahead and make use of rcu_assign_pointer() for both task
writes as it will make the __rcu sparse people happy - this will
be the special nil case, thus no added serialization.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Message-Id: <20200424054837.5138-4-dave@stgolabs.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>