bpf: allow sk_msg programs to read sock fields

Currently sk_msg programs only have access to the raw data. However,
it is often useful when building policies to have the policies specific
to the socket endpoint. This allows using the socket tuple as input
into filters, etc.

This patch adds ctx access to the sock fields.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/filter.h

@@ -517,6 +517,7 @@ struct sk_msg_buff {
 	bool sg_copy[MAX_SKB_FRAGS];
 	__u32 flags;
 	struct sock *sk_redir;
+	struct sock *sk;
 	struct sk_buff *skb;
 	struct list_head list;
 };