sched: Simplify migration_cpu_stop()

Since, when ->stop_pending, only the stopper can uninstall
p->migration_pending. This could simplify a few ifs, because:

  (pending != NULL) => (pending == p->migration_pending)

Also, the fatty comment above affine_move_task() probably needs a bit
of gardening.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>