Do not include private headers and do not frob in internals.

On top of that, while the previous code restores the affinity, it
doesn't ensure the task actually moves there if it was running,
leading to the fun situation that it can be observed running outside
of its allowed mask for potentially significant time.

Use the proper API instead.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/YG7QkiUzlEbW85TU@hirez.programming.kicks-ass.netSigned-off-by: NJens Axboe <axboe@kernel.dk>