f2fs: compress: do sanity check on cluster when CONFIG_F2FS_CHECK_FS is on

This patch covers sanity check logic on cluster w/ CONFIG_F2FS_CHECK_FS,
otherwise, there will be performance regression while querying cluster
mapping info.

Callers of f2fs_is_compressed_cluster() only care about whether cluster
is compressed or not, rather than # of valid blocks in compressed cluster,
so, let's adjust f2fs_is_compressed_cluster()'s logic according to
caller's requirement.

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