btrfs: use KMEM_CACHE() to create btrfs_ordered_extent cache

Use the KMEM_CACHE() macro instead of kmem_cache_create() to simplify
the creation of SLAB caches when the default values are used.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>