KVM: x86/mmu: Handle guest page faults for guest_memfd with shared memory

KVM: x86/mmu: Handle guest page faults for guest_memfd with shared memory

Update the KVM MMU fault handler to service guest page faults
for memory slots backed by guest_memfd with mmap support. For such
slots, the MMU must always fault in pages directly from guest_memfd,
bypassing the host's userspace_addr.

This ensures that guest_memfd-backed memory is always handled through
the guest_memfd specific faulting path, regardless of whether it's for
private or non-private (shared) use cases.

Additionally, rename kvm_mmu_faultin_pfn_private() to
kvm_mmu_faultin_pfn_gmem(), as this function is now used to fault in
pages from guest_memfd for both private and non-private memory,
accommodating the new use cases.

Co-developed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Co-developed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
[sean: drop the helper]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20250729225455.670324-17-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>