selftests/bpf: test_sockmap, check test failure

Test failures are not identified because exit code of RX/TX threads
is not checked. Also threads are not returning correct exit code.

- Return exit code from threads depending on test execution status
- In main thread, check the exit code of RX/TX threads
- Skip error checking for corked tests as they are expected to timeout

Fixes: 16962b24 ("bpf: sockmap, add selftests")
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>