ktrace/kern_exec: check p_tracecred instead of p_cred

.. when deciding whether to continue tracing across suid/sgid exec.
Otherwise if root ktrace-d an unprivileged process and the processed
exec-ed a suid program, then tracing didn't continue across exec.

Reviewed by:	bde, kib
MFC after:	22 days

sys/kern/kern_exec.c

@@ -691,7 +691,8 @@ do_execve(td, args, mac_p)
 		setsugid(p);
 
 #ifdef KTRACE
-		if (priv_check_cred(oldcred, PRIV_DEBUG_DIFFCRED, 0))
+		if (p->p_tracecred != NULL &&
+		    priv_check_cred(p->p_tracecred, PRIV_DEBUG_DIFFCRED, 0))
 			ktrprocexec(p, &tracecred, &tracevp);
 #endif
 		/*