lib/btree.c: fix leak of whole btree nodes

commit c75b53af2f0043aff500af0a6f878497bef41bca upstream.

I use btree from 3.14-rc2 in my own module.  When the btree module is
removed, a warning arises:

 kmem_cache_destroy btree_node: Slab cache still has objects
 CPU: 13 PID: 9150 Comm: rmmod Tainted: GF          O 3.14.0-rc2 #1
 Hardware name: Inspur NF5270M3/NF5270M3, BIOS CHEETAH_2.1.3 09/10/2013
 Call Trace:
   dump_stack+0x49/0x5d
   kmem_cache_destroy+0xcf/0xe0
   btree_module_exit+0x10/0x12 [btree]
   SyS_delete_module+0x198/0x1f0
   system_call_fastpath+0x16/0x1b

The cause is that it doesn't release the last btree node, when height = 1
and fill = 1.

Change-Id: I501d355755bb734d9574aa1c88d86c62f307c7a0