x86: Don't ENABLE_IBRS in nmi when we are still running on user cr3

x86: Don't ENABLE_IBRS in nmi when we are still running on user cr3

It won't end well - especially as we need to be careful about
touching kernel variables and can only do that in the kernel cr3.

The code:
        movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp

may lead one to believe you can access kernel variables, but in fact
we haven't yet switched over the kernel cr3.

Orabug: 27344012
CVE: CVE-2017-5715

Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>