Quantize szad trees by size class.

Treat sizes that round down to the same size class as size-equivalent
in trees that are used to search for first best fit, so that there are
only as many "firsts" as there are size classes.  This comes closer to
the ideal of first fit.

src/base.c

@@ -73,7 +73,7 @@ void *
 base_alloc(size_t size)
 {
 	void *ret;
-	size_t csize;
+	size_t csize, usize;
 	extent_node_t *node;
 	extent_node_t key;
 
@@ -83,7 +83,8 @@ base_alloc(size_t size)
 	 */
 	csize = CACHELINE_CEILING(size);
 
-	extent_node_init(&key, NULL, NULL, csize, false);
+	usize = s2u(csize);
+	extent_node_init(&key, NULL, NULL, usize, false);
 	malloc_mutex_lock(&base_mtx);
 	node = extent_tree_szad_nsearch(&base_avail_szad, &key);
 	if (node != NULL) {