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: b95a5c4d ("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>