Remove process information leak from inside a jail, when

security.bsd.see_other_uids is set to 0, etc.
One can check if invisible process is active, by doing:

	# ktrace -p <pid>

If ktrace returns 'Operation not permitted' the process is alive and
if returns 'No such process' there is no such process.

MFC after:	1 week

sys/kern/kern_ktrace.c

@@ -607,11 +607,14 @@ ktrace(td, uap)
 			error = ESRCH;
 			goto done;
 		}
+		error = p_cansee(td, p);
 		/*
 		 * The slock of the proctree lock will keep this process
 		 * from going away, so unlocking the proc here is ok.
 		 */
 		PROC_UNLOCK(p);
+		if (error)
+			goto done;
 		if (descend)
 			ret |= ktrsetchildren(td, p, ops, facs, vp);
 		else