afs: security: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

The use of "rcu_assign_pointer()" is NULLing out the pointer.
According to RCU_INIT_POINTER()'s block comment:
"1.   This use of RCU_INIT_POINTER() is NULLing out the pointer"
it is better to use it instead of rcu_assign_pointer() because it has a
smaller overhead.

The following Coccinelle semantic patch was used:
@@
@@

- rcu_assign_pointer
+ RCU_INIT_POINTER
  (..., NULL)
Signed-off-by: Andreea-Cristina Bernat <bernat.ada@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/security.c

@@ -114,7 +114,7 @@ void afs_clear_permits(struct afs_vnode *vnode)
 
 	mutex_lock(&vnode->permits_lock);
 	permits = vnode->permits;
-	rcu_assign_pointer(vnode->permits, NULL);
+	RCU_INIT_POINTER(vnode->permits, NULL);
 	mutex_unlock(&vnode->permits_lock);
 
 	if (permits)