rcuwait: Let rcuwait_wake_up() return whether or not a task was awoken

Propagating the return value of wake_up_process() back to the caller
can come in handy for future users, such as for statistics or
accounting purposes.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Message-Id: <20200424054837.5138-3-dave@stgolabs.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>