net_sched: fix qdisc_pkt_len_init()

commit 1def9238d4aa2 (net_sched: more precise pkt_len computation)
does a wrong computation of mac + network headers length, as it includes
the padding before the frame.

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