Fix yet another "unconditionally free()ing memory without even checking

to see if it was malloc()ed first" bug. In bus_dmamap_create(), one of
two things can happen: either we need to allocate a special map due to
some manner of bounce buffering requirement, or we can DMA a buffer
in place. On the x86 platform, the "in place" case results in
bus_dmamap_create() returning a dmamap of NULL. The bus_dmamap_destroy()
routine later checks for NULL and won't bother free()ing the map if
it detects this condition.

But on the alpha, we don't use NULL, we use a statically allocated map
called nobounce_dmamap(). Unfortunately, bus_dmamap_destroy() does not
handle the condition where we attempt to destroy such a map: it tries
to free() the dmamap, which causes a panic.

Fix: test that map != &nobounce_dmamap before trying to free() it.

With this fix, my busdma-ified if_sis driver works on the alpha. I'm
a bit alarmed that I'm the first person ever to trip over this bug, since
we have been using busdma on the alpha for a while, and since it sort
of screams out "Hi! I'm a bug! Booga-booga!" when you look at it.

(Somewhere, somebody will say: "But Bill, why don't you just not bother
destroying the maps in this case." Because the API is supposed to be
a) symetrical and b) opaque to the caller. I can't know whether it's safe
to skip the bus_dmamap_destroy() step or not without sticking my fingers
into unsafe places, which is what I wanted to avoid in the first place.)

sys/alpha/alpha/busdma_machdep.c

@@ -327,7 +327,13 @@ bus_dmamap_destroy(bus_dma_tag_t dmat, bus_dmamap_t map)
 	if (map != NULL) {
 		if (STAILQ_FIRST(&map->bpages) != NULL)
 			return (EBUSY);
-		free(map, M_DEVBUF);
+		/*
+		 * The nobounce_dmamap map is not dynamically
+		 * allocated, thus we should on no account try to
+		 * free it.
+		 */
+		if (map != &nobounce_dmamap)
+			free(map, M_DEVBUF);
 	}
 	dmat->map_count--;
 	return (0);