f2fs: introduce F2FS_INODE macro to get f2fs_inode

This patch introduces F2FS_INODE that returns struct f2fs_inode * from the inode
page.
By using this macro, we can remove unnecessary casting codes like below.

   struct f2fs_inode *ri = &F2FS_NODE(inode_page)->i;
-> struct f2fs_inode *ri = F2FS_INODE(inode_page);

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index ba91186..1a06f0a 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -498,6 +498,11 @@
 	return (struct f2fs_node *)page_address(page);
 }
 
+static inline struct f2fs_inode *F2FS_INODE(struct page *page)
+{
+	return &((struct f2fs_node *)page_address(page))->i;
+}
+
 static inline struct f2fs_nm_info *NM_I(struct f2fs_sb_info *sbi)
 {
 	return (struct f2fs_nm_info *)(sbi->nm_info);