tcp: bpf: Introduce bpf_sol_tcp_getsockopt to support TCP_BPF flags

The patch refactors a bit on supporting getsockopt for TCP BPF flags.
For now, only TCP_BPF_SOCK_OPS_CB_FLAGS. Later, more flags will be added
into this function.

No functional changes here.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250312153523.9860-2-kerneljasonxing@gmail.com