KVM: x86: Add helper to consolidate "raw" reserved GPA mask calculations

mainline inclusion
from mainline-v5.12-rc1
commit a8ac864a
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=a8ac864a7d6dbc2fc43081b1eecd9e0183065d47



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Add a helper to generate the mask of reserved GPA bits _without_ any
adjustments for repurposed bits, and use it to replace a variety of
open coded variants in the MTRR and APIC_BASE flows.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210204000117.3303214-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>