bpf: add get_next_key callback to LPM map

map_get_next_key callback is mandatory. Supply dummy handler.

Fixes: b95a5c4d ("bpf: add a longest prefix match trie map implementation")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>