qsbr urcu: make it safe to call rcu_sychronize from a registered thread.

Else, since calling rcu_sychronize will increment urcu_gp_ctr, we will be
stupidly waiting for ourselves and deadlock.

No need to use _rcu_quiescent_state() since we are currently owning the
internal URCU lock, so the memory barriers and ACCESS_ONCE() are overkill.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>