KVM: x86/mmu: Rename cr2_or_gpa to gpa in fast_page_fault

fast_page_fault is only called from direct_page_fault where we know the
address is a gpa.

Fixes: 736c291c ("KVM: x86: Use gpa_t for cr2/gpa to fix TDP support on 32-bit KVM")
Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210713220957.3493520-2-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>