iommu/amd: Argument page_size could be uninitialized

Argument "page_size" passing to function "fetch_pte" could be
uninitialized if the function returns NULL

The caller "iommu_unmap_page" checks the return value but the page_size is
used outside the if block.

Signed-off-by: yzhai003@ucr.edu <yzhai003@ucr.edu>
Signed-off-by: Joerg Roedel <jroedel@suse.de>