skmsg: Get rid of struct sk_psock_parser

struct sk_psock_parser is embedded in sk_psock, it is
unnecessary as skb verdict also uses ->saved_data_ready.
We can simply fold these fields into sk_psock, and get rid
of ->enabled.

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210223184934.6054-3-xiyou.wangcong@gmail.com