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: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>