xen: statically initialize cpu_evtchn_mask_p

Sometimes cpu_evtchn_mask_p can get used early, before it has been
allocated.  Statically initialize it with an initdata version to catch
any early references.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>