KVM: x86: avoid write-tearing of TDP

In theory, nothing prevents the compiler from write-tearing PTEs, or
split PTE writes. These partially-modified PTEs can be fetched by other
cores and cause mayhem. I have not really encountered such case in
real-life, but it does seem possible.

For example, the compiler may try to do something creative for
kvm_set_pte_rmapp() and perform multiple writes to the PTE.

Signed-off-by: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>