Btrfs: add helper btrfs_fdatawrite_range

To avoid duplicating this double filemap_fdatawrite_range() call for
inodes with async extents (compressed writes) so often.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index fe69edd..b72b358 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -3901,6 +3901,7 @@
 		      struct page **pages, size_t num_pages,
 		      loff_t pos, size_t write_bytes,
 		      struct extent_state **cached);
+int btrfs_fdatawrite_range(struct inode *inode, loff_t start, loff_t end);
 
 /* tree-defrag.c */
 int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,