bpf: Move the "cdg" tcp-cc check to the common sol_tcp_sockopt()

The check on the tcp-cc, "cdg", is done in the bpf_sk_setsockopt which is
used by the bpf_tcp_ca, bpf_lsm, cg_sockopt, and tcp_iter hooks.
However, it is not done for cg sock_ddr, cg sockops, and some of
the bpf_lsm_cgroup hooks.

The tcp-cc "cdg" should have very limited usage.  This patch is to
move the "cdg" check to the common sol_tcp_sockopt() so that all
hooks have a consistent behavior.   The motivation to make
this check consistent now is because the latter patch will
refactor the bpf_setsockopt(TCP_CONGESTION) into another function,
so it is better to take this chance to refactor this piece
also.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20220929070407.965581-3-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>