rcu: Allow calling rcu_(un)register_thread() during synchronize_rcu()

rcu: Allow calling rcu_(un)register_thread() during synchronize_rcu()

If rcu_(un)register_thread() is called together with synchronize_rcu(),
it will wait for the synchronize_rcu() to finish. But when synchronize_rcu()
waits for some events, we can modify the list registry.
We also use the lock rcu_gp_lock to assume that synchronize_rcu() isn't
executed in more than one thread at the same time. Add a new mutex lock
rcu_sync_lock to assume it and rename rcu_gp_lock to rcu_registry_lock.
Release rcu_registry_lock when synchronize_rcu() waits for some events.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Message-Id: <55B59652.4090503@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>