Fix a bug where DMA maps created with bus_dmamap_create() won't increment

the map count and without being able to keep track of the current map
allocation, bus_dma_tag_destroy() will fail to proceed and will return
EBUSY even after all the maps have been correctly destroyed with
bus_dmamap_destroy().

Found while testing the detach method of a NIC.

Tested on:	BBB (am335x)
Reviewed by:	cognet, ian
MFC after:	1 week

sys/arm/arm/busdma_machdep-v6.c

@@ -719,6 +719,8 @@ bus_dmamap_create(bus_dma_tag_t dmat, int flags, bus_dmamap_t *mapp)
 	if (map->flags & DMAMAP_COHERENT)
 		atomic_add_32(&maps_coherent, 1);
 	atomic_add_32(&maps_total, 1);
+	dmat->map_count++;
+
 	return (0);
 }