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
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.

[[e-mail]: remove unneeded test of NULL]
Signed-off-by: Minfei Huang <[e-mail]>
Cc: Joern Engel <[e-mail]>
Cc: Johannes Berg <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/btree.c

···
 
 void btree_destroy(struct btree_head *head)
 {
+	mempool_free(head->node, head->mempool);
 	mempool_destroy(head->mempool);
 	head->mempool = NULL;
 }