xen/arm: fix smpboot barriers

xen/arm: fix smpboot barriers

Remove useless smp_wmb() barrier after cpumask_set_cpu(cpuid,
&cpu_online_map), which is not synchronizing against anything.

Keep the other smp_wmb(), before the cpumask_set_cpu call, to ensure
that all writes before setting the cpu online are visible to other cpus.
For that to work properly, we need a corresponding smp_rmb() barrier,
after reading the online cpumask from other processors, which is
currently missing. Add it.

See: http://marc.info/?l=xen-devel&m=148093236307211

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>