KVM: nSVM: CR3 MBZ bits are only 63:52

Commit 761e4169 created a wrong mask for the
CR3 MBZ bits. According to APM vol 2, only the upper 12 bits are MBZ.

Fixes: 761e4169

 ("KVM: nSVM: Check that MBZ bits in CR3 and CR4 are not set on vmrun of nested guests", 2020-07-08)
Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Message-Id: <20200829004824.4577-2-krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>