Btrfs: Add readpages support

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_map.h

@@ -140,6 +140,10 @@ int extent_writepages(struct extent_map_tree *tree,
 		      struct address_space *mapping,
 		      get_extent_t *get_extent,
 		      struct writeback_control *wbc);
+int extent_readpages(struct extent_map_tree *tree,
+		     struct address_space *mapping,
+		     struct list_head *pages, unsigned nr_pages,
+		     get_extent_t get_extent);
 int extent_prepare_write(struct extent_map_tree *tree,
 			 struct inode *inode, struct page *page,
 			 unsigned from, unsigned to, get_extent_t *get_extent);