KVM: x86/mmu: Optimize not-present/MMIO SPTE check in get_mmio_spte()

mainline inclusion
from mainline-v5.11-rc3
commit 9aa41879
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I7S3VQ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9aa418792f5f11ef5d6f72265e1f8ae07efd5784



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Check only the terminal leaf for a "!PRESENT || MMIO" SPTE when looking
for reserved bits on valid, non-MMIO SPTEs.  The get_walk() helpers
terminate their walks if a not-present or MMIO SPTE is encountered, i.e.
the non-terminal SPTEs have already been verified to be regular SPTEs.
This eliminates an extra check-and-branch in a relatively hot loop.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20201218003139.2167891-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

conflict:
	arch/x86/kvm/mmu/mmu.c

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>