rcutorture: Add early boot self tests

Add early boot self tests for RCU under CONFIG_PROVE_RCU.

Currently the only test is adding a dummy callback which increments a counter
which we then later verify after calling rcu_barrier*().

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/rcu.h

@@ -135,4 +135,6 @@ int rcu_jiffies_till_stall_check(void);
  */
 #define TPS(x)  tracepoint_string(x)
 
+void rcu_early_boot_tests(void);
+
 #endif /* __LINUX_RCU_H */