KVM: selftests: Ensure gmem file sizes are multiple of host page size

KVM: selftests: Ensure gmem file sizes are multiple of host page size

When creating a guest_memfd file and associated memslot to validate shared
guest memory, size the file+memslot to the maximum of the host or guest
page size.  Attempting to allocate a single guest page will fail if the
host page size is greater than the guest page size, as KVM requires that
the size of memslots and guest_memfd files are a multiple of the host page
size.

For simplicity, verify the entire file can be shared between guest and host,
e.g. instead of trying to validate "partial" mappings.

Fixes: 42188667be38 ("KVM: selftests: Add guest_memfd testcase to fault-in on !mmap()'d memory")
Reported-by: Zenghui Yu <zenghui.yu@linux.dev>
Closes: https://lore.kernel.org/all/0064952b-048c-455d-ad89-e27e5cb82591@linux.dev
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260512155634.772602-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>