bpf: selftests: Add tests for batched ops in LPM trie maps

Uses the already existing infrastructure for testing batched ops.
The testing code is essentially the same, with minor tweaks for this use
case.

Suggested-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210323025058.315763-3-pctammela@gmail.com