KVM: guest_memfd: return folio from __kvm_gmem_get_pfn()

Right now this is simply more consistent and avoids use of pfn_to_page()
and put_page().  It will be put to more use in upcoming patches, to
ensure that the up-to-date flag is set at the very end of both the
kvm_gmem_get_pfn() and kvm_gmem_populate() flows.

Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>