ANDROID: KVM: arm64: iommu: Fix upper bound of PT walk

The second argument of the kvm_pgtable_walker callback was
misinterpreted as the end of the current entry, where in fact it is
the end of the walked memory region. Fix this by computing the end of
the current entry from the start and the level.

This did not affect correctness, as the code iterates linarly over
the entire address space, but it did affect boot time.

Bug: 190463801
Bug: 218012133
Signed-off-by: David Brazdil <dbrazdil@google.com>
Change-Id: I6d189b87645f47cd215a783c1bc9e1f032ff8c62