bridge: Restart queries when last querier expires

As it stands when we discover that a real querier (one that queries
with a non-zero source address) we stop querying.  However, even
after said querier has fallen off the edge of the earth, we will
never restart querying (unless the bridge itself is restarted).

This patch fixes this by kicking our own querier into gear when
the timer for other queriers expire.

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