Don't dereference chunk->arena in free() hot path

When you call free() we load chunk->arena even though that
data isn't used on the tcache hot path.

In profiling some FB applications, I found that ~30% of the
dTLB misses in the free() function come from this line. With
4 MB chunks, the arena_chunk_t->map is ~ 32 KB (1024 pages
in the chunk, 4 8 byte pointers in arena_chunk_map_t). This
means there's only a 1/8 chance of the page containing
chunk->arena also comtaining the map bits.

src/jemalloc.c

@@ -2103,7 +2103,7 @@ a0free(void *ptr)
 
 	chunk = (arena_chunk_t *)CHUNK_ADDR2BASE(ptr);
 	if (chunk != ptr)
-		arena_dalloc(chunk->arena, chunk, ptr, false);
+		arena_dalloc(chunk, ptr, false);
 	else
 		huge_dalloc(ptr, true);
 }