Currently we arm the expire timer when the mdb entry is added,
however, this causes problem when there is no querier sent
out after that.

So we should only arm the timer when a corresponding query is
received, as suggested by Herbert.

And he also mentioned "if there is no querier then group
subscriptions shouldn't expire. There has to be at least one querier
in the network for this thing to work.  Otherwise it just degenerates
into a non-snooping switch, which is OK."

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: NCong Wang <amwang@redhat.com>
Acked-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>