selftests/bpf: Check bpf_sk_storage has uncharged sk_omem_alloc

This patch checks the sk_omem_alloc has been uncharged by bpf_sk_storage
during the __sk_destruct.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230901231129.578493-4-martin.lau@linux.dev