Whenever the kernel checks for rt_task() to avoid delaying real-time
tasks, we want it to also not delay LITMUS^RT tasks.  Hence, most
calls to rt_task() should be matched by an equivalent call to
is_realtime(). This patch fixes a few places where this was missing.
Notably, this affects the impolementation of select(), which uses
timer_slack_ns when setting up timers for non-real-time tasks.

As with SCHED_FIFO/SCHED_RR tasks, artificial latency is not
desired...

(BB: edited to include <litmus/litmus.h> to resolve compile error.)

Linux's post_schedule() scheduling class hook more closely matches
what SCHED2 is supposed to trace, namely any scheduling overhead after
the context switch. The prior trace points caught timers being armed
from finish_switch(), which is already included in the context switch
cost CXS.

(This patch essentially reverts 8fe2fb8bb1c1cd0194608bc783d0ce7029e8d869).

IPIs have some special cases where irq_enter() is not called. This
caused ft_irq_fired() to "miss" some rescheduling-related interrupts,
which in turn may cause outliers.

This patch makes sure ft_irq_fired() is called on scheduling-related
IPIs.

We don't want outliers due to soft IRQs, so let them mark ongoing
traces as "dirty" as well.

The LITMUS^RT-specific completion API complete_n() is no longer
required by the synchronous release code. Let's remove it; one less
modification of a core Linux file to maintain during rebasing.

Fixes a bug in Litmus where processor scheduling states
could become corrupted. Corruption can occur when a
just-forked thread is externally forced to be scheduled
by SCHED_LITMUS before this just-forked thread can complete
post-fork processing. Specifically, before schedule_tail()
has completed.

Enable kernel-style events (tracepoint) for Litmus. Litmus events
trace the same functions as the sched_trace_XXX(), but can be
enabled independently.

So, why another tracing infrastructure then:
- Litmus tracepoints can be recorded and analyzed together (single
  time reference) with all other kernel tracing events (e.g.,
  sched:sched_switch, etc.). It's easier to correlate the effects
  of kernel events on litmus tasks.

- It enables a quick way to visualize and process schedule traces
  using trace-cmd utility and kernelshark visualizer.
  Kernelshark lacks unit-trace's schedule-correctness checks, but
  it enables a fast view of schedule traces and it has several
  filtering options (for all kernel events, not only Litmus').