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

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

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
ffff881ff8643b18 ffff881ffdc23ea8 ffffffff815a4ecc 0000000000000000
ffff881ff8643ac0 ffff881ffdc23ec8 ffffffff811610df 0000000000000880
ffffffffa057da60 ffff881ffdc23ed8 ffffffffa057d57c ffff881ffdc23f78
Call Trace:
[<ffffffff815a4ecc>] dump_stack+0x49/0x5d
[<ffffffff811610df>] kmem_cache_destroy+0xcf/0xe0
[<ffffffffa057d57c>] btree_module_exit+0x10/0x12 [btree]
[<ffffffff810d7948>] SyS_delete_module+0x198/0x1f0
[<ffffffff815aac89>] ? retint_swapgs+0xe/0x13
[<ffffffff810a561d>] ? trace_hardirqs_on_caller+0xfd/0x1c0
[<ffffffff812addde>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[<ffffffff815b3652>] system_call_fastpath+0x16/0x1b

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

Signed-off-by: Minfei Huang <huangminfei@ucloud.cn>
Cc: Joern Engel <joern@logfs.org>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>