In addition to the periodic heartbeat, we're adding a heartbeat
request interrupt when the heartbeat is late.  This is needed during
netpoll where the timer is not available.  -rt kernels will also
benefit since the timer is not as accurate.

[ We discussed this patch last time and we decided that the -rt
  kernel problem alone did not justify this patch.  I think the
  netpoll problem makes this patch necessary. ]
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>