KVM: guest_memfd: fix NUMA interleave index double-counting

kvm_gmem_get_policy() sets *ilx to the full page offset
(vm_pgoff + vma offset).  But get_vma_policy() adds the page
offset on top of *ilx, so the offset is counted twice.  This
causes NUMA interleaving to skip nodes: for order-0 pages the
effective index jumps by 2 for each consecutive page.

The get_policy vm_op should return only a per-file bias in *ilx
(like shmem_get_policy does with inode->i_ino), letting
get_vma_policy() add the page-offset component.

Fix by setting *ilx to inode->i_ino instead of the full page
offset.  The page offset is computed by get_vma_policy() in
mm/mempolicy.c. The full offset is still computed
in kvm_gmem_get_policy() for mpol_shared_policy_lookup().
shmem_get_policy() follows the same pattern.

Found by Sashiko (sashiko.dev) AI code review.

Fixes: ed1ffa810bd6 ("KVM: guest_memfd: Enforce NUMA mempolicy using shared policy")
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>