KVM: x86/mmu: Handle no-slot faults at the beginning of kvm_faultin_pfn()

Handle the "no memslot" case at the beginning of kvm_faultin_pfn(), just
after the private versus shared check, so that there's no need to
repeatedly query whether or not a slot exists.  This also makes it more
obvious that, except for private vs. shared attributes, the process of
faulting in a pfn simply doesn't apply to gfns without a slot.

Opportunistically stuff @fault's metadata in kvm_handle_noslot_fault() so
that it doesn't need to be duplicated in all paths that invoke
kvm_handle_noslot_fault(), and to minimize the probability of not stuffing
the right fields.

Leave the existing handle behind, but convert it to a WARN, to guard
against __kvm_faultin_pfn() unexpectedly nullifying fault->slot.

Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Message-ID: <20240228024147.41573-14-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>