selftests/bpf: Extend test_pkt_access test

The test_pkt_access.o is used by multiple tests. Fix its section name so that
program type can be automatically detected by libbpf and make it call other
subprograms with skb argument.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191114185720.1641606-20-ast@kernel.org