Apply the logic from r363051 to semctl(2) and __sem_base field.

Reported by:	Jeffball <jeffball@grimm-co.com>
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25600

lib/libc/sys/semctl.2

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd February 23, 2018
+.Dd July 9, 2020
 .Dt SEMCTL 2
 .Os
 .Sh NAME
@@ -148,7 +148,6 @@ is defined as follows:
 .Bd -literal
 struct semid_ds {
         struct  ipc_perm sem_perm;      /* operation permission struct */
-        struct  sem *__sem_base;  /* kernel data, don't use */
         u_short sem_nsems;      /* number of sems in set */
         time_t  sem_otime;      /* last operation time */
         time_t  sem_ctime;      /* last change time */

sys/kern/sysv_sem.c

@@ -798,6 +798,13 @@ kern_semctl(struct thread *td, int semid, int semnum, int cmd,
 		bcopy(&semakptr->u, arg->buf, sizeof(struct semid_ds));
 		if (cred->cr_prison != semakptr->cred->cr_prison)
 			arg->buf->sem_perm.key = IPC_PRIVATE;
+
+		/*
+		 * Try to hide the fact that the structure layout is shared by
+		 * both the kernel and userland.  This pointer is not useful to
+		 * userspace.
+		 */
+		arg->buf->__sem_base = NULL;
 		break;
 
 	case GETNCNT: