rcu: create rcu_my_thread_group_empty() wrapper

Some RCU-lockdep splat repairs need to know whether they are running
in a single-threaded process.  Unfortunately, the thread_group_empty()
primitive is defined in sched.h, and can induce #include hell.  This
commit therefore introduces a rcu_my_thread_group_empty() wrapper that
is defined in rcupdate.c, thus avoiding the need to include sched.h
everywhere.

Signed-off-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

include/linux/rcupdate.h

@@ -190,6 +190,8 @@ static inline int rcu_read_lock_sched_held(void)
 
 #ifdef CONFIG_PROVE_RCU
 
+extern int rcu_my_thread_group_empty(void);
+
 /**
  * rcu_dereference_check - rcu_dereference with debug checking
  * @p: The pointer to read, prior to dereferencing