xen: events: mark cpu_evtchn_mask_p as __refdata

This variable starts out pointing at init_evtchn_mask which is marked
__initdata but is set to point to a non-init data region in xen_init_IRQ
which is itself an __init function so this is safe.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Tested-and-acked-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>