xen/evtchn: use smp barriers for user event ring

The ring buffer for user events is local to the given kernel instance,
so smp barriers are fine for ensuring consistency.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Link: https://lore.kernel.org/r/20210219154030.10892-8-jgross@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>