Moved sched_litmus back to the top scheduling class

It is now stop -> dl -> rt -> litmus -> fair

1. Adds a dummy balance function to LITMUS^RT and e-enables Linux's task balancing code.
2. Removes usage of `prev` from `pick_next_task_litmus()`
3. Removes duplicate call to `put_prev_task()` from `pick_next_task_litmus()` (this has been moved into core.c for all schedulers)
4. Fixes an unguarded use of `prev` in the `schedule()` function for SCHED_LITMUS

 - Fixed a bug where some new balancing code wasn't commented out when
   it needed to be. (It wasn't in the version of Linux that the previous
   version of LITMUS was based on, so I missed it earlier.)

 - The GSN-EDF plugin can now be activated, and runs tasks without
   crashing (though I haven't verified that it schedules stuff in the
   correct order yet).

 - *All remaining plugins probably need to be modified in the same was
   as the GSN-EDF plugin*. Specifically, the "prev" argument to the
   "<plugin>_schedule(...)" function may be NULL, and the plugin must be
   able to handle such a case!

 - Since the last version of LITMUS, the pick_next_task function for
   scheduler classes has changed a bit; now the "prev" argument can be
   NULL. This change fixes how LITMUS' scheduler class stubs handle
   this, but further attention may need to be given to fix the
   non-default LITMUS plugins.

 - This change addresses additional changes that are needed to LITMUS
   code so that the kernel is able to compile.

 - The kernel is now able to compile.

 - I will probably squash this commit with the previous commit sometime
   later, so that all of the LITMUS-updating changes are in one commit
   and easy to see, *if* this kernel works.

 - This is my first attempt to re-add all of the modifications on top of
   this version of the Linux kernel that were present in the previous
   version of LITMUS.

 - More notes on changes will follow after testing--no guarantees the
   code as it is now will compile or run correctly.