x86: merge 64 and 32 SMP percpu handling

Now that pda is allocated as part of percpu, percpu doesn't need to be
accessed through pda.  Unify x86_64 SMP percpu access with x86_32 SMP
one.  Other than the segment register, operand size and the base of
percpu symbols, they behave identical now.

This patch replaces now unnecessary pda->data_offset with a dummy
field which is necessary to keep stack_canary at its place.  This
patch also moves per_cpu_offset initialization out of init_gdt() into
setup_per_cpu_areas().  Note that this change also necessitates
explicit per_cpu_offset initializations in voyager_smp.c.

With this change, x86_OP_percpu()'s are as efficient on x86_64 as on
x86_32 and also x86_64 can use assembly PER_CPU macros.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/asm-offsets_64.c

@@ -55,7 +55,6 @@ int main(void)
 	ENTRY(irqcount);
 	ENTRY(cpunumber);
 	ENTRY(irqstackptr);
-	ENTRY(data_offset);
 	DEFINE(pda_size, sizeof(struct x8664_pda));
 	BLANK();
 #undef ENTRY