f2fs: fix to clear PG_checked flag in set_page_dirty()

PG_checked flag will be set on data page during GC, later, we can
recognize such page by the flag and migrate page to cold segment.

But previously, we don't clear this flag when invalidating data page,
after page redirtying, we will write it into wrong log.

Let's clear PG_checked flag in set_page_dirty() to avoid this.

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