Idle regexp eats up softirq frames

[bobrik]

We borrowed idle_regexp for our internal system and it looks like the regexp hides some legitimate stacks in softirq context.

Current filter produces the following flamegraph:

If I make idle_regexp match nothing, I can see much more interesting stuff:

The trace comes from on Intel Skylake.

[spiermar]

@brendangregg ^^

[spiermar]

@brendangregg can you take a look into the idle regex used to filter idle stacks? There's a clear example of the stack filtering differences in #143