bpf, lpm: fix kfree of im_node in trie_update_elem

We need to initialize im_node to NULL, otherwise in case of error path
it gets passed to kfree() as uninitialized pointer.

Fixes: b95a5c4db09b ("bpf: add a longest prefix match trie map implementation")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/lpm_trie.c

@@ -262,7 +262,7 @@ static int trie_update_elem(struct bpf_map *map,
 			    void *_key, void *value, u64 flags)
 {
 	struct lpm_trie *trie = container_of(map, struct lpm_trie, map);
-	struct lpm_trie_node *node, *im_node, *new_node = NULL;
+	struct lpm_trie_node *node, *im_node = NULL, *new_node = NULL;
 	struct lpm_trie_node __rcu **slot;
 	struct bpf_lpm_trie_key *key = _key;
 	unsigned long irq_flags;