vfio/ccw: Add kmap_local_page() for memcpy

vfio/ccw: Add kmap_local_page() for memcpy

A PFN is not secure enough to promise that the memory is not IO. And
direct access via memcpy() that only handles CPU memory will crash on
S390 if the PFN is an IO PFN, as we have to use the memcpy_to/fromio()
that uses the special S390 IO access instructions. On the other hand,
a "struct page *" is always a CPU coherent thing that fits memcpy().

Also, casting a PFN to "void *" for memcpy() is not a proper practice,
kmap_local_page() is the correct API to call here, though S390 doesn't
use highmem, which means kmap_local_page() is a NOP.

There's a following patch changing the vfio_pin_pages() API to return
a list of "struct page *" instead of PFNs. It will block any IO memory
from ever getting into this call path, for such a security purpose. In
this patch, add kmap_local_page() to prepare for that.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Eric Farman <farman@linux.ibm.com>
Tested-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Link: https://lore.kernel.org/r/20220723020256.30081-10-nicolinc@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>