fix lguest rmmod "bad pgd"

After 17d57a92 ("x86: fix x86-32 early
fixmap initialization.") removing lg.ko caused a printk from vunmap:

	mm/memory.c:115: bad pgd 004b3027.

On the second use after module load, the kernel crashes.

This fixes the immediate problem (accessed and dirty bits not set as
expected in pmd_none_or_clear_bad).  I can't see why this would cause
a crash, but I haven't been able to reproduce it once this is applied.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>