btrfs: move basic block_group definitions to their own header

This is prep work for moving all of the block group cache code into its
own file.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor comment updates ]
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/free-space-tree.h b/fs/btrfs/free-space-tree.h
index 22b7602..360d50e 100644
--- a/fs/btrfs/free-space-tree.h
+++ b/fs/btrfs/free-space-tree.h
@@ -6,6 +6,8 @@
 #ifndef BTRFS_FREE_SPACE_TREE_H
 #define BTRFS_FREE_SPACE_TREE_H
 
+struct btrfs_caching_control;
+
 /*
  * The default size for new free space bitmap items. The last bitmap in a block
  * group may be truncated, and none of the free space tree code assumes that