evtchn: double per-channel locking can't hit identical channels

evtchn: double per-channel locking can't hit identical channels

Inter-domain channels can't possibly be bound to themselves, there's
always a 2nd channel involved, even when this is a loopback into the
same domain. As a result we can drop one conditional each from the two
involved functions.

With this, the number of evtchn_write_lock() invocations can also be
shrunk by half, swapping the two incoming function arguments instead.

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