f2fs: compress: don't compress any datas after cp stop

While compressed data writeback, we need to drop dirty pages like we did
for non-compressed pages if cp stops, however it's not needed to compress
any data in such case, so let's detect cp stop condition in
cluster_may_compress() to avoid redundant compressing and let following
f2fs_write_raw_pages() drops dirty pages correctly.

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