lguest: accept guest _PAGE_PWT page table entries

Beginning from commit 4138cc34

, ioremap_nocache() sets the _PAGE_PWT
flag.

Lguest doesn't accept a guest pte with a _PWT flag and reports a "bad
page table entry" in that case.

Accept guest _PAGE_PWT page table entries.

Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>