x86/mm/64: Fix reboot interaction with CR4.PCIDE

Trying to reboot via real mode fails with PCID on: long mode cannot
be exited while CR4.PCIDE is set.  (No, I have no idea why, but the
SDM and actual CPUs are in agreement here.)  The result is a GPF and
a hang instead of a reboot.

I didn't catch this in testing because neither my computer nor my VM
reboots this way.  I can trigger it with reboot=bios, though.

Fixes: 660da7c9 ("x86/mm: Enable CR4.PCIDE on supported systems")
Reported-and-tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/f1e7d965998018450a7a70c2823873686a8b21c0.1507524746.git.luto@kernel.org