aya-bpf: expose sk_msg_md of SkMsgContext

This patch expose sk_msg_md of SkMsgContext.

bpf/aya-bpf/src/programs/sk_msg.rs

@@ -7,7 +7,7 @@ use crate::{
 };
 
 pub struct SkMsgContext {
-    msg: *mut sk_msg_md,
+    pub msg: *mut sk_msg_md,
 }
 
 impl SkMsgContext {