Well in this you get into a circular argument, because if you did include Linux then you would need to verify every line of code that is being used. Which is why you would never use linux and why it wouldn't be included.
*This is in reference to DAL A software. DAL D might be able to run linux.
But if you want to write a sensational "100 million lines of code" then you count the code that's running the in flight entertainment system (and others) which doesn't have the same safety constraints.
The code would most like still be counted but that software would probably be either DAL D or DAL E.
Here is a the wiki for the standard that software has to meet for commercial aircraft. It explain the DAL (Design Assurance Level) levels and what goes into them.
https://en.wikipedia.org/wiki/DO-178C
