KVM: x86/mmu: Pass full 64-bit error code when handling page faults

Plumb the full 64-bit error code throughout the page fault handling code
so that KVM can use the upper 32 bits, e.g. SNP's PFERR_GUEST_ENC_MASK
will be used to determine whether or not a fault is private vs. shared.

Note, passing the 64-bit error code to FNAME(walk_addr)() does NOT change
the behavior of permission_fault() when invoked in the page fault path, as
KVM explicitly clears PFERR_IMPLICIT_ACCESS in kvm_mmu_page_fault().

Continue passing '0' from the async #PF worker, as guest_memfd and thus
private memory doesn't support async page faults.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
[mdr: drop references/changes on rebase, update commit message]
Signed-off-by: Michael Roth <michael.roth@amd.com>
[sean: drop truncation in call to FNAME(walk_addr)(), rewrite changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20240228024147.41573-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>