f2fs: compress: remove unneeded read when rewrite whole cluster

when we overwrite the whole page in cluster, we don't need read original
data before write, because after write_end(), writepages() can help to
load left data in that cluster.

Signed-off-by: Fengnan Chang <changfengnan@vivo.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>