iommu/pages: Formalize the freelist API

iommu/pages: Formalize the freelist API

We want to get rid of struct page references outside the internal
allocator implementation. The free list has the driver open code something
like:

   list_add_tail(&virt_to_page(ptr)->lru, freelist);

Move the above into a small inline and make the freelist into a wrapper
type 'struct iommu_pages_list' so that the compiler can help check all the
conversion.

This struct has also proven helpful in some future ideas to convert to a
singly linked list to get an extra pointer in the struct page, and to
signal that the pages should be freed with RCU.

Use a temporary _Generic so we don't need to rename the free function as
the patches progress.

Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/8-v4-c8663abbb606+3f7-iommu_pages_jgg@nvidia.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>