bpf: Add networking timestamping support to bpf_get/setsockopt()

The new SK_BPF_CB_FLAGS and new SK_BPF_CB_TX_TIMESTAMPING are
added to bpf_get/setsockopt. The later patches will implement the
BPF networking timestamping. The BPF program will use
bpf_setsockopt(SK_BPF_CB_FLAGS, SK_BPF_CB_TX_TIMESTAMPING) to
enable the BPF networking timestamping on a socket.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250220072940.99994-2-kerneljasonxing@gmail.com