net_sched: htb: do not mix 1ns and 64ns time units

commit 56b765b7

 ("htb: improved accuracy at high rates") added another
regression for low rates, because it mixes 1ns and 64ns time units.

So the maximum delay (mbuffer) was not 60 second, but 937 ms.

Lets convert all time fields to 1ns as 64bit arches are becoming the
norm.

Reported-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>