f2fs: introduce help function F2FS_NODE()

Introduce help function F2FS_NODE() to simplify the conversion of node_page to
f2fs_node.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

diff --cc fs/f2fs/data.c
Simple merge

diff --cc fs/f2fs/dir.c
Simple merge

diff --cc fs/f2fs/f2fs.h
Simple merge

diff --cc fs/f2fs/file.c
Simple merge

diff --cc fs/f2fs/inode.c
Simple merge

diff --cc fs/f2fs/node.c
Simple merge

diff --cc fs/f2fs/node.h
Simple merge

diff --cc fs/f2fs/recovery.c
Simple merge