SC: use SC_LG_NGROUP instead of its value.

This magic constant introduces inconsistencies.  We should be able to change its
value solely by adjusting the definition in the header.

src/sc.c

@@ -262,7 +262,7 @@ sc_data_init(sc_data_t *sc_data) {
 	int lg_max_lookup = 12;
 
 	size_classes(sc_data, LG_SIZEOF_PTR, LG_QUANTUM, SC_LG_TINY_MIN,
-	    lg_max_lookup, LG_PAGE, 2);
+	    lg_max_lookup, LG_PAGE, SC_LG_NGROUP);
 
 	sc_data->initialized = true;
 }