selftests/bpf: Improve test coverage of test_maps

Make test_maps more stressful with more parallelism in
update/delete/lookup/walk including different value sizes.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220902211058.60789-4-alexei.starovoitov@gmail.com