tools/bpf: add a multithreaded stress test in bpf selftests test_lpm_map

The new test will spawn four threads, doing map update, delete, lookup
and get_next_key in parallel. It is able to reproduce the issue in the
previous commit found by syzbot and Eric Dumazet.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>