The probability of guest_memfd not having struct page for mapped pfns is likely
very low, but at the same time, providing a pfn+page pair doesn't cost us much.
And if it turns out that not having struct page is nonsensical, deferring the
kvm_gmem_get_pfn() => kvm_gmem_get_page() conversion could be annoying, but highly
unlikely to be painful since it should be 100% mechanical. Whereas reverting back
to kvm_gmem_get_pfn() if we make the wrong decision now could mean doing surgery
on a pile of arch code.