pkt_sched: fq: clear time_next_packet for reused flows

When a socket is freed/reallocated, we need to clear time_next_packet
or else we can inherit a prior value and delay first packets of the
new flow.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>