Restore assertion that was removed in r182444.

While merging back my changes from Perforce, it seemed I removed a
locking assertion that still applies to ttydisc_rint_poll(). Restore it,
because it may come in handy.

sys/sys/ttydisc.h

@@ -86,6 +86,8 @@ ttydisc_rint_poll(struct tty *tp)
 {
 	size_t l;
 
+	tty_lock_assert(tp, MA_OWNED);
+
 	/*
 	 * XXX: Still allow character input when there's no space in the
 	 * buffers, but we haven't entered the high watermark. This is