KVM: arm64: Fix vma_shift staleness on nested hwpoison path

When user_mem_abort() handles a nested stage-2 fault, it truncates
vma_pagesize to respect the guest's mapping size. However, the local
variable vma_shift is never updated to match this new size.

If the underlying host page turns out to be hardware poisoned,
kvm_send_hwpoison_signal() is called with the original, larger
vma_shift instead of the actual mapping size. This signals incorrect
poison boundaries to userspace and breaks hugepage memory poison
containment for nested VMs.

Update vma_shift to match the truncated vma_pagesize when operating
on behalf of a nested hypervisor.

Fixes: fd276e71d1e7 ("KVM: arm64: nv: Handle shadow stage 2 page faults")
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260304162222.836152-3-tabba@google.com
[maz: simplified vma_shift assignment from the original patch]
Signed-off-by: Marc Zyngier <maz@kernel.org>