Btrfs: use a slab for the free space entries

Since we alloc/free free space entries a whole lot, lets use a slab to keep
track of them.  This makes some of my tests slightly faster.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 7f78cc7..2c98d20 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -40,6 +40,7 @@
 extern struct kmem_cache *btrfs_transaction_cachep;
 extern struct kmem_cache *btrfs_bit_radix_cachep;
 extern struct kmem_cache *btrfs_path_cachep;
+extern struct kmem_cache *btrfs_free_space_cachep;
 struct btrfs_ordered_sum;
 
 #define BTRFS_MAGIC "_BHRfS_M"