bpf: fix rcu lockdep warning for lpm_trie map_free callback

Commit 9a3efb6 ("bpf: fix memory leak in lpm_trie map_free callback function")
fixed a memory leak and removed unnecessary locks in map_free callback function.
Unfortrunately, it introduced a lockdep warning. When lockdep checking is turned on,
running tools/testing/selftests/bpf/test_lpm_map will have:

  [   98.294321] =============================
  [   98.294807] WARNING: suspicious RCU usage
  [   98.295359] 4.16.0-rc2+ torvalds#193 Not tainted
  [   98.295907] -----------------------------
  [   98.296486] /home/yhs/work/bpf/kernel/bpf/lpm_trie.c:572 suspicious rcu_dereference_check() usage!
  [   98.297657]
  [   98.297657] other info that might help us debug this:
  [   98.297657]
  [   98.298663]
  [   98.298663] rcu_scheduler_active = 2, debug_locks = 1
  [   98.299536] 2 locks held by kworker/2:1/54:
  [   98.300152]  #0:  ((wq_completion)"events"){+.+.}, at: [<00000000196bc1f0>] process_one_work+0x157/0x5c0
  [   98.301381]  #1:  ((work_completion)(&map->work)){+.+.}, at: [<00000000196bc1f0>] process_one_work+0x157/0x5c0

Since actual trie tree removal happens only after no other
accesses to the tree are possible, this patch simply converted all
rcu protected pointer access to normal access, which removed the
above warning.

Fixes: 9a3efb6 ("bpf: fix memory leak in lpm_trie map_free callback function")
Reported-by: Eric Dumazet <[email protected]>
Signed-off-by: Yonghong Song <[email protected]>

kernel/bpf/lpm_trie.c

@@ -552,7 +552,7 @@ static struct bpf_map *trie_alloc(union bpf_attr *attr)
 static void trie_free(struct bpf_map *map)
 {
 	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
-	struct lpm_trie_node __rcu **slot;
+	struct lpm_trie_node **slot;
 	struct lpm_trie_node *node;
 
 	/* Wait for outstanding programs to complete
@@ -569,23 +569,22 @@ static void trie_free(struct bpf_map *map)
 		slot = &trie->root;
 
 		for (;;) {
-			node = rcu_dereference_protected(*slot,
-					lockdep_is_held(&trie->lock));
+			node = *slot;
 			if (!node)
 				goto out;
 
-			if (rcu_access_pointer(node->child[0])) {
+			if (node->child[0]) {
 				slot = &node->child[0];
 				continue;
 			}
 
-			if (rcu_access_pointer(node->child[1])) {
+			if (node->child[1]) {
 				slot = &node->child[1];
 				continue;
 			}
 
 			kfree(node);
-			RCU_INIT_POINTER(*slot, NULL);
+			*slot = NULL;
 			break;
 		}
 	}