KVM: nSVM: Use common GPA helper to check for illegal CR3

mainline inclusion
from mainline-v5.12-rc1
commit bbc2c63d
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=bbc2c63ddd51cd6d349e3fe0010f9b7b259e58ea



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Replace an open coded check for an invalid CR3 with its equivalent
helper.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210204000117.3303214-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>