f2fs: compress: add sanity check during compressed cluster read

In f2fs_read_multi_pages(), we don't have to check cluster's type
again, since overwrite or partial truncation need page lock in
cluster which has already been held by reader, so cluster's type
is stable, let's change check condition to sanity check.

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