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/relocation.c b/fs/btrfs/relocation.c
index 300e09a..ba5a321 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3017,7 +3017,7 @@
 			}
 		}
 
-		page_start = (u64)page->index << PAGE_CACHE_SHIFT;
+		page_start = page_offset(page);
 		page_end = page_start + PAGE_CACHE_SIZE - 1;
 
 		lock_extent(&BTRFS_I(inode)->io_tree, page_start, page_end);