urcu call_rcu: Use RCU read-side protection for per-cpu call_rcu data

urcu call_rcu: Use RCU read-side protection for per-cpu call_rcu data

A concurrent get_cpu_call_rcu_data(), called by get_call_rcu_data(),
could dereference this pointer without holding any mutex. So this
situation would happen if we have a concurrent call_rcu() executing
while we do the create_all_cpu_call_rcu_data().

I think we would need to put a rcu_dereference() around
per_cpu_call_rcu_data read within get_cpu_call_rcu_data() too.
per_cpu_call_rcu_data should be done with rcu_set_pointer.

Also, a rcu read-side critical section would be required around any
usage of per_cpu_call_rcu_data, and the action of tearing down the
per-cpu data would require to wait for a quiescent state. So we would
basically require that the call_rcu users need to be registered as
RCU reader threads.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>