sched/psi: export psi_memstall_{enter,leave}

To properly account for all refaults from file system logic, file systems
need to call psi_memstall_enter directly, so export it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20220915094200.139713-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>