bpf: Add ene-to-end test for bpf_sk_storage_* helpers

This patch rides on an existing BPF_PROG_TYPE_CGROUP_SKB test
(test_sock_fields.c) to do a TCP end-to-end test on the new
bpf_sk_storage_* helpers.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>