f2fs: fix to reorder set_page_dirty and wait_on_page_writeback

This patch reorders flow from

- update page
- set_page_dirty
- wait_on_page_writeback

to

- wait_on_page_writeback
- update page
- set_page_dirty

The reason is:
- set_page_dirty will increase reference of dirty page, the reference
should be cleared before wait_on_page_writeback to keep its consistency.
- some devices need stable page during page writebacking, so we
should not change page's data.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>