vfio: powerpc/spapr: Moving pinning/unpinning to helpers

This is a pretty mechanical patch to make next patches simpler.

New tce_iommu_unuse_page() helper does put_page() now but it might skip
that after the memory registering patch applied.

As we are here, this removes unnecessary checks for a value returned
by pfn_to_page() as it cannot possibly return NULL.

This moves tce_iommu_disable() later to let tce_iommu_clear() know if
the container has been enabled because if it has not been, then
put_page() must not be called on TCEs from the TCE table. This situation
is not yet possible but it will after KVM acceleration patchset is
applied.

This changes code to work with physical addresses rather than linear
mapping addresses for better code readability. Following patches will
add an xchg() callback for an IOMMU table which will accept/return
physical addresses (unlike current tce_build()) which will eliminate
redundant conversions.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[aw: for the vfio related changes]
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>