x86/iopl/64: properly context-switch IOPL on Xen PV

x86/iopl/64: properly context-switch IOPL on Xen PV

On Xen PV, regs->flags doesn't reliably reflect IOPL and the
exit-to-userspace code doesn't change IOPL.  We need to context
switch it manually.

I'm doing this without going through paravirt because this is
specific to Xen PV.  After the dust settles, we can merge this with
the 32-bit code, tidy up the iopl syscall implementation, and remove
the set_iopl pvop entirely.

This is XSA-171.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: stable@vger.kernel.org
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Orabug: 22926124
Conflicts:
  arch/x86/kernel/process_64.c
  X86_FEATURE_XENPV not defined, too much extra to bring it in.
  Replaced with xen_pv_domain() which is trigger to set X86_FEATURE_XENPV.
CVE: CVE-2016-3157
Signed-off-by: Chuck Anderson <chuck.anderson@oracle.com>