x86, apic: Use PAGE_SIZE instead of numbers

The whole page is reserved for IO-APIC fixmap
due to non-cacheable requirement. So lets note
this explicitly instead of playing with numbers.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
LKML-Reference: <20091108155356.GB25940@lenovo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>