srcu: Allow a second bit in rcu_seq for SRCU state

This commit increases the number of reserved bits at the bottom of an
rcu_seq grace-period counter from one to two, as will be needed to
accommodate SRCU's three-state grace periods.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcu.h

@@ -61,7 +61,7 @@
  * Grace-period counter management.
  */
 
-#define RCU_SEQ_CTR_SHIFT	1
+#define RCU_SEQ_CTR_SHIFT	2
 #define RCU_SEQ_STATE_MASK	((1 << RCU_SEQ_CTR_SHIFT) - 1)
 
 /*