arena: fix qcache double-free

This was nasty.  The qcache would be freed twice, which meant that it
would get reused twice.  It resulted in the full_slab_list having weird
shit on it: it looked like a SLIST of magazines!

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 5a006d9..cbf1711 100644
--- a/kern/src/arena.c
+++ b/kern/src/arena.c
@@ -174,7 +174,7 @@
 
 	for (int i = 0; i < nr_qcaches; i++) {
 		kc = &arena->qcaches[i];
-		kmem_cache_destroy(kc);
+		__kmem_cache_destroy(kc);
 	}
 
 	base_free(arena, arena->qcaches,