lint: Fix current clippy errors

[dcantah]

error: the borrowed expression implements the required traits
   --> src/hierarchies.rs:179:30
    |
179 |         Cgroup::load(auto(), &parent_path)
    |                              ^^^^^^^^^^^^ help: change this to: `parent_path`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
    = note: `-D clippy::needless-borrow` implied by `-D warnings`

error: the borrowed expression implements the required traits
   --> src/hierarchies.rs:261:30
    |
261 |         Cgroup::load(auto(), &parent_path)
    |                              ^^^^^^^^^^^^ help: change this to: `parent_path`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow