Fix bitmap_sfu() regression.

Fix bitmap_sfu() to shift by LG_BITMAP_GROUP_NBITS rather than
hard-coded 6 when using linear (non-USE_TREE) bitmap search.  In
practice this affects only 64-bit systems for which sizeof(long) is not
8 (i.e. Windows), since USE_TREE is defined for 32-bit systems.

This regression was caused by b8823ab02607d6f03febd32ac504bb6188c54047
(Use linear scan for small bitmaps).

This resolves #368.

include/jemalloc/internal/bitmap.h

@@ -223,7 +223,7 @@ bitmap_sfu(bitmap_t *bitmap, const bitmap_info_t *binfo)
 		i++;
 		g = bitmap[i];
 	}
-	bit = (bit - 1) + (i << 6);
+	bit = (bit - 1) + (i << LG_BITMAP_GROUP_NBITS);
 #endif
 	bitmap_set(bitmap, binfo, bit);
 	return (bit);