net_sched: sch_fq: avoid calling ktime_get_ns() if not needed

There are two cases were we can avoid calling ktime_get_ns() :

1) Queue is empty.
2) Internal queue is not empty.

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