Put "failed to set signal flags properly for ast()" check under

DIAGNOSTIC instead of INVARIANTS.  INVARIANTS is intended for tests
that don't substantially change code flow or behavior (passive), but
this test required locking both the proc lock and scheduler lock
in order to execute.  It also appears to be a very advisory diagnostic
as opposed to an invariant violation.

Following discussion with:	bde

sys/kern/subr_trap.c

@@ -83,7 +83,7 @@ userret(td, frame, oticks)
 
 	CTR3(KTR_SYSC, "userret: thread %p (pid %d, %s)", td, p->p_pid,
             p->p_comm);
-#ifdef INVARIANTS
+#ifdef DIAGNOSTIC
 	/* Check that we called signotify() enough. */
 	PROC_LOCK(p);
 	mtx_lock_spin(&sched_lock);