sched: Handle blocked-waiter migration (and return migration)

Add logic to handle migrating a blocked waiter to a remote
cpu where the lock owner is runnable.

Additionally, as the blocked task may not be able to run
on the remote cpu, add logic to handle return migration once
the waiting task is given the mutex.

Because tasks may get migrated to where they cannot run, also
modify the scheduling classes to avoid sched class migrations on
mutex blocked tasks, leaving find_proxy_task() and related logic
to do the migrations and return migrations.

This was split out from the larger proxy patch, and
significantly reworked.

Credits for the original patch go to:
  Peter Zijlstra (Intel) <peterz@infradead.org>
  Juri Lelli <juri.lelli@redhat.com>
  Valentin Schneider <valentin.schneider@arm.com>
  Connor O'Brien <connoro@google.com>

Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260324191337.1841376-11-jstultz@google.com

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