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: NJeremy 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: NIngo Molnar <mingo@elte.hu>