KVM: x86/mmu: Add helper to generate mask of reserved HPA bits

mainline inclusion
from mainline-v5.12-rc1
commit 6f8e65a6
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=6f8e65a60168567cc59f9b99980ea9112d4152f5



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Add a helper to generate the mask of reserved PA bits in the host.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210204000117.3303214-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>