x86/spec_ctrl: save IBRS MSR value in paranoid_entry

x86/spec_ctrl: save IBRS MSR value in paranoid_entry

If the NMI runs while entering kernel between SWAPGS and IBRS_ENABLE
everything is fine, paranoid_entry would have unconditionally set
IBRS bit 0 and when exiting the NMI it would have cleared bit 0 like
if it was returning to userland. IBRS_ENABLE would have then enabled
bit 0 again.

If NMI instead runs when exiting kernel between IBRS_DISABLE and
SWAPGS, the NMI would have turned on IBRS bit 0 and then it would have
left enabled when exiting the NMI. IBRS bit 0 would then be left
enabled in userland until the next enter kernel.

That is a minor inefficiency only, but we can eliminate it by saving
the MSR when entering the NMI in save_paranoid and restoring it when
exiting the NMI.

Orabug: 27344012
CVE: CVE-2017-5715

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: John Haxby <john.haxby@oracle.com>
Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>