btrfs: open code __free_extent_buffer()

Using the kmem cache freeing directly is clear enough, we don't need to
wrap it.  All the users are in the same file.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>