KVM: x86/mmu: Move slot checks from __kvm_faultin_pfn() to kvm_faultin_pfn()

Move the checks related to the validity of an access to a memslot from the
inner __kvm_faultin_pfn() to its sole caller, kvm_faultin_pfn().  This
allows emulating accesses to the APIC access page, which don't need to
resolve a pfn, even if there is a relevant in-progress mmu_notifier
invalidation.  Ditto for accesses to KVM internal memslots from L2, which
KVM also treats as emulated MMIO.

More importantly, this will allow for future cleanup by having the
"no memslot" case bail from kvm_faultin_pfn() very early on.

Go to rather extreme and gross lengths to make the change a glorified
nop, e.g. call into __kvm_faultin_pfn() even when there is no slot, as the
related code is very subtle.  E.g. fault->slot can be nullified if it
points at the APIC access page, some flows in KVM x86 expect fault->pfn
to be KVM_PFN_NOSLOT, while others check only fault->slot, etc.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Message-ID: <20240228024147.41573-13-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>