evtchn: don't call Xen consumer callback with per-channel lock held

evtchn: don't call Xen consumer callback with per-channel lock held

While there don't look to be any problems with this right now, the lock
order implications from holding the lock can be very difficult to follow
(and may be easy to violate unknowingly). The present callbacks don't
(and no such callback should) have any need for the lock to be held.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>