srcu: Remove checks preventing offline CPUs from calling srcu_read_lock()

SRCU has its own statemachine and no longer relies on normal RCU.
Its read-side critical section can now be used by an offline CPU, so this
commit removes the check and the comments, reverting the SRCU portion
of c0d6d01b (rcu: Check for illegal use of RCU from offlined CPUs).

It also makes the code match the comments in whatisRCU.txt:

g.	Do you need read-side critical sections that are respected
	even though they are in the middle of the idle loop, during
	user-mode execution, or on an offlined CPU?  If so, SRCU is the
	only choice that will work for you.

[ paulmck: There is at least one remaining issue, namely use of lockdep
	   with tracing enabled. ]
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/srcu.h

@@ -163,9 +163,6 @@ void srcu_barrier(struct srcu_struct *sp);
  * power mode. This way we can notice an extended quiescent state to
  * other CPUs that started a grace period. Otherwise we would delay any
  * grace period as long as we run in the idle task.
- *
- * Similarly, we avoid claiming an SRCU read lock held if the current
- * CPU is offline.
  */
 static inline int srcu_read_lock_held(struct srcu_struct *sp)
 {
@@ -173,8 +170,6 @@ static inline int srcu_read_lock_held(struct srcu_struct *sp)
 		return 1;
 	if (rcu_is_cpu_idle())
 		return 0;
-	if (!rcu_lockdep_current_cpu_online())
-		return 0;
 	return lock_is_held(&sp->dep_map);
 }