x86, apic: Slim down stack usage in early_init_lapic_mapping()

As far as I see there is no external poking of mp_lapic_addr in
this procedure which could lead to unpredited changes and
require local storage unit for it. Lets use it plain forward.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <20090826171324.GC4548@lenovo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>