kvm: x86: Toggling CR4.SMAP does not load PDPTEs in PAE mode

See the SDM, volume 3, section 4.4.1:

If PAE paging would be in use following an execution of MOV to CR0 or
MOV to CR4 (see Section 4.1.1) and the instruction is modifying any of
CR0.CD, CR0.NW, CR0.PG, CR4.PAE, CR4.PGE, CR4.PSE, or CR4.SMEP; then
the PDPTEs are loaded from the address in CR3.

Fixes: 0be0226f ("KVM: MMU: fix SMAP virtualization")
Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Message-Id: <20200817181655.3716509-2-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>