safexcel: Stop using a stack buffer for the ring lock name

mtx_init() does not make a copy of the name so the buffer must be valid
for the lifetime of the driver instance.  Store each ring's lock's name
in the ring structure.

MFC after:	3 days
Sponsored by:	Rubicon Communications, LLC (Netgate)

sys/dev/safexcel/safexcel.c

@@ -783,15 +783,15 @@ safexcel_init_rings(struct safexcel_softc *sc)
 {
 	struct safexcel_cmd_descr *cdesc;
 	struct safexcel_ring *ring;
-	char buf[32];
 	uint64_t atok;
 	int i, j;
 
 	for (i = 0; i < sc->sc_config.rings; i++) {
 		ring = &sc->sc_ring[i];
 
-		snprintf(buf, sizeof(buf), "safexcel_ring%d", i);
-		mtx_init(&ring->mtx, buf, NULL, MTX_DEF);
+		snprintf(ring->lockname, sizeof(ring->lockname),
+		    "safexcel_ring%d", i);
+		mtx_init(&ring->mtx, ring->lockname, NULL, MTX_DEF);
 		STAILQ_INIT(&ring->free_requests);
 		STAILQ_INIT(&ring->ready_requests);
 		STAILQ_INIT(&ring->queued_requests);

sys/dev/safexcel/safexcel_var.h

@@ -384,6 +384,8 @@ struct safexcel_ring {
 
 	struct safexcel_dma_mem		dma_atok;
 	bus_dma_tag_t   		data_dtag;
+
+	char				lockname[32];
 };
 
 struct safexcel_intr_handle {