epoch: fix assertion

If we call epoch_exit_preempt() on an epoch that's not net_epoch_preempt
we can't assert td->td_net_epoch.
Move the assertion under the 'epoch == net_epoch_preempt' conditional.

No functional change for pfSense, as we don't build with INVARIANTS
(unless debugging, in which case this panicked during boot).

sys/kern/subr_epoch.c

@@ -377,9 +377,10 @@ epoch_exit_preempt(epoch_t epoch, epoch_tracker_t et)
 	td = curthread;
 	critical_enter();
 	sched_unpin();
-	MPASS(td->td_net_epoch);
-	if (epoch == net_epoch_preempt)
+	if (epoch == net_epoch_preempt) {
+		MPASS(td->td_net_epoch);
 		td->td_net_epoch--;
+	}
 	MPASS(td->td_epochnest);
 	td->td_epochnest--;
 	er = epoch_currecord(epoch);