[NET] linkwatch: Handle jiffies wrap-around

The test used in the linkwatch does not handle wrap-arounds correctly.
Since the intention of the code is to eliminate bursts of messages we
can afford to delay things up to a second.  Using that fact we can
easily handle wrap-arounds by making sure that we don't delay things
by more than one second.

This is based on diagnosis and a patch by Stefan Rompf.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Stefan Rompf <stefan@loplof.de>
Signed-off-by: David S. Miller <davem@davemloft.net>