Get current zswap memory usage and statistics

The [zswap documentation](https://www.kernel.org/doc/Documentation/vm/zswap.txt) says: > Zswap seeks to be simple in its policies. Sysfs attributes allow for one user controlled policy: * max_pool_percent - The maximum percentage of memory that the compressed pool can occupy. This specifies the *maximum* percentage of memory the compressed pool can occupy. How do I find out: * The *current* percentage of memory occupied by the compressed pool * How much of this pool is in use * Compression ratios, hit rates, and other useful info