Convert extent_t's usize to szind.

Rather than storing usize only for large (and prof-promoted)
allocations, store the size class index for allocations that reside
within the extent, such that the size class index is valid for all
extents that contain extant allocations, and invalid otherwise (mainly
to make debugging simpler).

include/jemalloc/internal/extent_structs.h

@@ -20,10 +20,12 @@ struct extent_s {
 	size_t			e_size;
 
 	/*
-	 * Usable size, typically smaller than extent size due to large_pad or
+	 * Usable size class index for allocations residing in this extent,
+	 * regardless of whether the extent is a slab.  Extent size and usable
+	 * size often differ even for non-slabs, either due to large_pad or
 	 * promotion of sampled small regions.
 	 */
-	size_t			e_usize;
+	szind_t			e_szind;
 
 	/*
 	 * Serial number (potentially non-unique).