ktls: fix a panic with INVARIANTS

9821500 introduced a new
thread that uses tsleep(..0) to sleep forever.  This hit
an assert due to sleeping with a 0 timeout.

So spell "forever" using SBT_MAX instead, which does not
trigger the assert.

Pointy hat to: gallatin
Pointed out by: emaste
Sponsored by: Netflix

sys/kern/uipc_ktls.c

@@ -2240,7 +2240,7 @@ ktls_alloc_thread(void *ctx)
 	nbufs = 0;
 	for (;;) {
 		atomic_store_int(&sc->running, 0);
-		tsleep(sc, PZERO, "waiting for work", 0);
+		tsleep_sbt(sc, PZERO, "waiting for work", SBT_MAX, SBT_1S, 0);
 		atomic_store_int(&sc->running, 1);
 		sc->wakeups++;
 		if (nbufs != ktls_max_alloc) {