arena: fix btag freeing in arena_destroy()

We are freeing the btags, not the objects they point to.

Signed-off-by: Barret Rhoden <brho@cs.berkeley.edu>

kern/src/arena.c

diff --git a/kern/src/arena.c b/kern/src/arena.c
index 00807ee..2c26ff3 100644
--- a/kern/src/arena.c
+++ b/kern/src/arena.c
@@ -239,12 +239,12 @@
 	 * The BTs that are page aligned are the ones we want.  We can just
 	 * ignore the others (unlink from the list). */
 	BSD_LIST_FOREACH_SAFE(bt_i, &arena->unused_btags, misc_link, temp) {
-		if (PGOFF(bt_i->start))
+		if (PGOFF(bt_i))
 			BSD_LIST_REMOVE(bt_i, misc_link);
 	}
 	/* Now the remaining BTs are the first on their page. */
 	BSD_LIST_FOREACH_SAFE(bt_i, &arena->unused_btags, misc_link, temp)
-		arena_free(find_my_base(arena), (void*)bt_i->start, PGSIZE);
+		arena_free(find_my_base(arena), bt_i, PGSIZE);
 	kfree(arena);
 }