bpf: Add hwtstamp field for the sockops prog

The bpf-tc prog has already been able to access the
skb_hwtstamps(skb)->hwtstamp.  This patch extends the same hwtstamp
access to the sockops prog.

In sockops, the skb is also available to the bpf prog during
the BPF_SOCK_OPS_PARSE_HDR_OPT_CB event.  There is a use case
that the hwtstamp will be useful to the sockops prog to better
measure the one-way-delay when the sender has put the tx
timestamp in the tcp header option.

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/20221107230420.4192307-2-martin.lau@linux.dev

include/uapi/linux/bpf.h

@@ -6445,6 +6445,7 @@ struct bpf_sock_ops {
 				 * the outgoing header has not
 				 * been written yet.
 				 */
+	__u64 skb_hwtstamp;
 };
 
 /* Definitions for bpf_sock_ops_cb_flags */