f2fs: use f2fs_get_node_page when write inline data

We just need inode page when write inline data, use
f2fs_get_node_page() to get it instead of using dnode_of_data,
which can eliminate unnecessary struct use.

Signed-off-by: Zijie Wang <wangzijie1@honor.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>