rcu: Set rcu_data structures' initial ->gpwrap value to true

It would be good do reduce the size of the rcu_gp_oldstate structure
from three unsigned long instances to two, but this requires that the
boot-time optimized grace periods update the various ->gp_seq fields.
Updating these fields in the rcu_state structure and in all of the
rcu_node structures is at least semi-reasonable, but updating them in
all of the rcu_data structures is a bridge too far.  This means that if
there are too many early boot-time grace periods, the ->gp_seq field in
the rcu_data structure cannot be trusted.  This commit therefore sets
each rcu_data structure's ->gpwrap field to provide the necessary impetus
for a suitable level of distrust.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -76,6 +76,7 @@
 /* Data structures. */
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
+	.gpwrap = true,
 #ifdef CONFIG_RCU_NOCB_CPU
 	.cblist.flags = SEGCBLIST_RCU_CORE,
 #endif