[NET] link_watch: Always schedule urgent events

Urgent events may be delayed if we already have a non-urgent event
queued for that device.  This patch changes this by making sure that
an urgent event is always looked at immediately.

I've replaced the LW_RUNNING flag by LW_URGENT since whether work
is scheduled is already kept track by the work queue system.

The only complication is that we have to provide some exclusion for
the setting linkwatch_nextevent which is available in the actual
work function.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>