KVM: pfncache: stop open-coding offset_in_page()

Some code in pfncache uses offset_in_page() but in other places it is open-
coded. Use offset_in_page() consistently everywhere.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20240215152916.1158-7-paul@xen.org


Signed-off-by: Sean Christopherson <seanjc@google.com>