f2fs: always unlock the page in f2fs_write_single_data_page

Consolidate the code to unlock the page in f2fs_write_single_data_page
instead of leaving it to the callers for the AOP_WRITEPAGE_ACTIVATE case.
Replace AOP_WRITEPAGE_ACTIVATE with a positive return of 1 as this case
now doesn't match the historic ->writepage special return code that is
on it's way out now that ->writepage has been removed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>