bpf: Refactor sock_ops_convert_ctx_access

The next patch will introduce a new "struct bpf_tcp_sock" which
exposes the same tcp_sock's fields already exposed in
"struct bpf_sock_ops".

This patch refactor the existing convert_ctx_access() codes for
"struct bpf_sock_ops" to get them ready to be reused for
"struct bpf_tcp_sock".  The "rtt_min" is not refactored
in this patch because its handling is different from other
fields.

The SOCK_OPS_GET_TCP_SOCK_FIELD is new. All other SOCK_OPS_XXX_FIELD
changes are code move only.
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>