selftests/bpf: test_progs/sockopt_sk: Convert to use BPF skeleton

Switch the test to use BPF skeleton to save some boilerplate and
make it easy to access bpf program bss segment.

The latter will be used to pass PAGE_SIZE from userspace since there
is no convenient way for bpf program to get it from inside of the
kernel.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210408061310.95877-2-yauheni.kaliuta@redhat.com