use 64-bit msqid_ds layout in the generic version of bits/msg.h

this layout is more common already than the old generic, and should
become even more common in the future with new archs added and with
64-bit time_t on 32-bit archs.

arch/generic/bits/msg.h

 struct msqid_ds {
 	struct ipc_perm msg_perm;
 	time_t msg_stime;
-	int __unused1;
 	time_t msg_rtime;
-	int __unused2;
 	time_t msg_ctime;
-	int __unused3;
 	unsigned long msg_cbytes;
 	msgqnum_t msg_qnum;
 	msglen_t msg_qbytes;