KVM: x86: make vapics_in_nmi_mode atomic

[ Upstream commit 42720138b06301cc8a7ee8a495a6d021c4b6a9bc ]

Writes were a bit racy, but hard to turn into a bug at the same time.
(Particularly because modern Linux doesn't use this feature anymore.)

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
[Actually the next patch makes it much, much easier to trigger the race
 so I'm including this one for stable@ as well. - Paolo]
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>