x86: Fix spectre/kpti integration

x86: Fix spectre/kpti integration

The issue is that DISABLE_IBRS (and pretty much all of the _IBRS) first
operation is touching an kernel variable. The restore_c_regs_and_iret is
already in user-space cr3 so we page fault.

The fix is simple - do not run any of the IBRS macros from within
restore_c_regs_and_iret. Which means that the three functions that
used to call it now have to call IBRS_DISABLE by themselves:
retint_swapgs, opportunistic_sysret_failed, and nmi.

Adding in the IBRS_DISABLE in opportunistic_sysret_failed also
fixes another bug - which is more clearly explained in
"x86/enter: Use IBRS on syscall and interrupts  - fix ia32 path"

Orabug: 27333760
CVE: CVE-2017-5754
Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>