libbpf: Support batch_size option to bpf_prog_test_run

Add support for setting the new batch_size parameter to BPF_PROG_TEST_RUN
to libbpf; just add it as an option and pass it through to the kernel.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220309105346.100053-4-toke@redhat.com