workaround broken msghdr struct on 64bit linux

POSIX clearly specifies the type of msg_iovlen and msg_controllen, and
Linux ignores it and makes them both size_t instead. to work around
this we add padding (instead of just using the wrong types like glibc
does), but we also need to patch-up the struct before passing it to
the kernel in case the caller did not zero-fill it.

if i could trust the kernel to just ignore the upper 32 bits, this
would not be necessary, but i don't think it will ignore them...

arch/x86_64/bits/socket.h

@@ -3,9 +3,9 @@ struct msghdr
 	void *msg_name;
 	socklen_t msg_namelen;
 	struct iovec *msg_iov;
-	int msg_iovlen;
+	int msg_iovlen, __pad1;
 	void *msg_control;
-	socklen_t msg_controllen;
+	socklen_t msg_controllen, __pad2;
 	int msg_flags;
 };