[NET] link_watch: Eliminate potential delay on wrap-around

When the jiffies wrap around or when the system boots up for the first
time, down events can be delayed indefinitely since we no longer
update linkwatch_nextevent when only urgent events are processed.

This patch fixes this by setting linkwatch_nextevent when a
wrap-around occurs.

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