fix socket.h struct msghdr member types on powerpc

these were incorrectly copied from the kernel, whose ABI matches the
POSIX requirements but with the wrong underlying types and wrong
signedness.

arch/powerpc/bits/socket.h

@@ -1,12 +1,12 @@
 struct msghdr
 {
 	void *msg_name;
-	int msg_namelen;
+	socklen_t msg_namelen;
 	struct iovec *msg_iov;
-	unsigned long msg_iovlen;
+	int msg_iovlen;
 	void *msg_control;
-	unsigned long  msg_controllen;
-	unsigned msg_flags;
+	socklen_t msg_controllen;
+	int msg_flags;
 };
 
 struct cmsghdr