af_unix: Convert the unix_sk macro to an inline function for type safety

As suggested by Eric Dumazet this change replaces the
#define with a static inline function to enjoy
complaints by the compiler when misusing the API.

Signed-off-by: Aaron Conole <aconole@bytheb.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: apconole

include/net/af_unix.h

@@ -63,7 +63,11 @@ struct unix_sock {
 #define UNIX_GC_MAYBE_CYCLE	1
 	struct socket_wq	peer_wq;
 };
-#define unix_sk(__sk) ((struct unix_sock *)__sk)
+
+static inline struct unix_sock *unix_sk(struct sock *sk)
+{
+	return (struct unix_sock *)sk;
+}
 
 #define peer_wait peer_wq.wait