rcu: Better explain the condition parameter of rcu_dereference_check()

/**

 * rcu_dereference_check - rcu_dereference with debug checking

 * @p: The pointer to read, prior to dereferencing

 * @c: The conditions under which the dereference will take place

 *

 * Do an rcu_dereference(), but check that the context is correct.

 * For example, rcu_dereference_check(gp, rcu_read_lock_held()) to

 * ensure that the rcu_dereference_check() executes within an RCU

 * read-side critical section.  It is also possible to check for

 * locks being held, for example, by using lockdep_is_held().

 * Do an rcu_dereference(), but check that the conditions under which the

 * dereference will take place are correct.  Typically the conditions indicate

 * the various locking conditions that should be held at that point.  The check

 * should return true if the conditions are satisfied.

 *

 * For example:

 *

 *	bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||

 *					      lockdep_is_held(&foo->lock));

 *

 * could be used to indicate to lockdep that foo->bar may only be dereferenced

 * if either the RCU read lock is held, or that the lock required to replace

 * the bar struct at foo->bar is held.

 *

 * Note that the list of conditions may also include indications of when a lock

 * need not be held, for example during initialisation or destruction of the

 * target struct:

 *

 *	bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() ||

 *					      lockdep_is_held(&foo->lock) ||

 *					      atomic_read(&foo->usage) == 0);

 */

#define rcu_dereference_check(p, c) \

	({ \