f2fs: compress: avoid duplicate counting of valid blocks when read compressed file

Since cluster is basic unit of compression, one cluster is compressed or
not, so we can calculate valid blocks only for first page in cluster,
the other pages just skip.

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