Btrfs: use wrapper page_offset

Use wrapper page_offset to get byte-offset into filesystem object for page.

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 1ea0988..35d1524 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -2062,7 +2062,7 @@
 static int btrfs_readpage_end_io_hook(struct page *page, u64 start, u64 end,
 			       struct extent_state *state, int mirror)
 {
-	size_t offset = start - ((u64)page->index << PAGE_CACHE_SHIFT);
+	size_t offset = start - page_offset(page);
 	struct inode *inode = page->mapping->host;
 	struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
 	char *kaddr;
@@ -6752,8 +6752,7 @@
 		return;
 	}
 	lock_extent_bits(tree, page_start, page_end, 0, &cached_state);
-	ordered = btrfs_lookup_ordered_extent(inode,
-					   page_offset(page));
+	ordered = btrfs_lookup_ordered_extent(inode, page_offset(page));
 	if (ordered) {
 		/*
 		 * IO on this page will never be started, so we need