Revert "f2fs: fix to clear PG_checked flag in set_page_dirty()"

This reverts commit 66110abc.

If we clear the cold data flag out of the writeback flow, we can miscount
-1 by end_io, which incurs a deadlock caused by all I/Os being blocked during
heavy GC.

Balancing F2FS Async:
 - IO (CP:    1, Data:   -1, Flush: (   0    0    1), Discard: (   ...

GC thread:                              IRQ
- move_data_page()
 - set_page_dirty()
  - clear_cold_data()
                                        - f2fs_write_end_io()
                                         - type = WB_DATA_TYPE(page);
                                           here, we get wrong type
                                         - dec_page_count(sbi, type);
 - f2fs_wait_on_page_writeback()

Cc: <stable@vger.kernel.org>
Reported-and-Tested-by: Park Ju Hyung <qkrwngud825@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>