Add non-panicking variants to `cgroups_rs::memory::MemController::memory_stat`

[sisungo]

Which feature do you think can be improved? When cgroup2 filesystem is unmounted as the program is running and the method is called later, the method triggers a panic!(). The panic is located at .unwrap() at line 587 in memory.rs.

This behavior is not documented and may make things like writing the init daemon have an unexpected behavior.

How can it be improved? This issue request to add a non-panicking variant to methods like this. The non-panicking methods may have names like try_xxx() and returns a Result<_>.

[sisungo]

Duplicates with #116.