ipc: fix unused variable warning

Commit a0d092fc introduced the following warning:
ipc/msg.c: In function ?msgctl_down?:
ipc/msg.c:415: warning: ?msqid64? may be used uninitialized in this function

The gcc warning in this case is actually bogus, as msqid64 is touched only
iff cmd == IPC_SET, and in such case, copy_msqid_from_user() initializes
it properly.
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

ipc/msg.c

@@ -412,7 +412,7 @@ static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
 		       struct msqid_ds __user *buf, int version)
 {
 	struct kern_ipc_perm *ipcp;
-	struct msqid64_ds msqid64;
+	struct msqid64_ds uninitialized_var(msqid64);
 	struct msg_queue *msq;
 	int err;