ia64: sn_nodepda cannot be assigned to after this_cpu conversion. Use __this_cpu_write.

There must be an explit statement to modify the percpu variable after
the conversion of the sn_nodpda macro to use this_cpu_read.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Compile-tested-by: Guenter Roeck <linux@roeck-us.net>