Btrfs: prepare block group cache before writing

Writing the block group cache will modify the extent tree quite a bit because it
truncates the old space cache and pre-allocates new stuff.  To try and cut down
on the churn lets do the setup dance first, then later on hopefully we can avoid
looping with newly dirtied roots.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index b3dd55f..a0c90a3 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -3386,6 +3386,8 @@
 
 int btrfs_write_dirty_block_groups(struct btrfs_trans_handle *trans,
 				    struct btrfs_root *root);
+int btrfs_setup_space_cache(struct btrfs_trans_handle *trans,
+			    struct btrfs_root *root);
 int btrfs_extent_readonly(struct btrfs_root *root, u64 bytenr);
 int btrfs_free_block_groups(struct btrfs_fs_info *info);
 int btrfs_read_block_groups(struct btrfs_root *root);