x86-32: make sure virt_addr_valid() returns false for fixmap addresses

I found that virt_addr_valid() was returning true for fixmap addresses.

I'm not sure whether pfn_valid() is supposed to include this test,
but there's no harm in being explicit.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Jiri Slaby <jirislaby@gmail.com>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <49B166D6.2080505@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>