rcu: Update rcu_access_pointer() header for rcu_dereference_protected()

 * against NULL.  Although rcu_access_pointer() may also be used in cases

 * where update-side locks prevent the value of the pointer from changing,

 * you should instead use rcu_dereference_protected() for this use case.

 * Within an RCU read-side critical section, there is little reason to

 * use rcu_access_pointer().

 *

 * It is usually best to test the rcu_access_pointer() return value

 * directly in order to avoid accidental dereferences being introduced

 * by later inattentive changes.  In other words, assigning the

 * rcu_access_pointer() return value to a local variable results in an

 * accident waiting to happen.

 *

 * It is also permissible to use rcu_access_pointer() when read-side

 * access to the pointer was removed at least one grace period ago, as

 * is the case in the context of the RCU callback that is freeing up

 * the data, or after a synchronize_rcu() returns.  This can be useful

 * when tearing down multi-linked structures after a grace period

 * has elapsed.

 * access to the pointer was removed at least one grace period ago, as is

 * the case in the context of the RCU callback that is freeing up the data,

 * or after a synchronize_rcu() returns.  This can be useful when tearing

 * down multi-linked structures after a grace period has elapsed.  However,

 * rcu_dereference_protected() is normally preferred for this use case.

 */

#define rcu_access_pointer(p) __rcu_access_pointer((p), __UNIQUE_ID(rcu), __rcu)