Adding sync_shrinkers function for TTM pool optimization

Hi Andrew,

Daniel suggested that I ping you once more about this.

Basically we want to add a barrier function to make sure that our TTM pool shrinker is not freeing up pages from a device while the device is being unplugged.

Currently we are having a global mutex to serialize all of this, but this caused contention for unmapping the freed pages in the IOMMU.

We just need your Acked-by and I hope my explanation is now more understandable than the last time.

Cheers, Christian.