f2fs: fix to update page flag

This patch fixes to update page flag (e.g. Uptodate/cold flag) in
->write_begin.

Otherwise, page will be non-uptodate when we try to write entire
page, and cold data flag in page will not be clean when gced page
is being rewritten.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>