ionic: avoid races in ionic_heartbeat_check

Rework the heartbeat checks to be sure that we're getting an
atomic operation.  Through testing we found occasions where a
separate thread could clash with this check and cause erroneous
heartbeat check results.

Signed-off-by: Allen Hubbe <allenbh@pensando.io>
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>