KVM: x86/mmu: Check for host MMIO exclusion from mem encrypt iff necessary

When determining whether or not a SPTE needs to have SME/SEV's memory
encryption flag set, do the moderately expensive host MMIO pfn check if
and only if the memory encryption mask is non-zero.

Note, KVM could further optimize the host MMIO checks by making a single
call to kvm_is_mmio_pfn(), but the tdp_enabled path (for EPT's memtype
handling) will likely be split out to a separate flow[*].  At that point,
a better approach would be to shove the call to kvm_is_mmio_pfn() into
VMX code so that AMD+NPT without SME doesn't get hit with an unnecessary
lookup.

[*] https://lkml.kernel.org/r/[email protected]

Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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