Reuse regex when parsing proc mountinfo

[robinkrahl]

Instead of compiling a the same regex once for every line in the
mountinfo file, we can instead reuse the same regex for all lines.

[robinkrahl]

It would also be possible to use the same regex for all invocations (example), but multiple invocations are an unlikely use case and that would add additional dependencies (lazy_static, once_cell or similar). Therefore I think this change is a good trade-off between runtime and compile time costs.