sch_sfq: fix sfq_enqueue()

commit 8efa8854 (sch_sfq: avoid giving spurious NET_XMIT_CN signals)
forgot to call qdisc_tree_decrease_qlen() to signal upper levels that a
packet (from another flow) was dropped, leading to various problems.

With help from Michal Soltys and Michal Pokrywka, who did a bisection.

Bugzilla ref: https://bugzilla.kernel.org/show_bug.cgi?id=39372
Debian ref: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=631945



Reported-by: Lucas Bocchi <lucas.bocchi@gmail.com>
Reported-and-bisected-by: Michal Pokrywka <wolfmoon@o2.pl>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Michal Soltys <soltys@ziu.info>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>