selftests/bpf: Test skops->skb_hwtstamp

This patch tests reading the skops->skb_hwtstamp field.

A local test was also done such that the shinfo hwtstamp was temporary
set to a non zero value in the kernel bpf_skops_parse_hdr()
and the same value can be read by the skops test.

An adjustment is needed to the btf_dump selftest because
the changes in the 'struct bpf_sock_ops'.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221107230420.4192307-4-martin.lau@linux.dev