sch_htb: Remove local SKB queue handling code.

Instead, adjust __qdisc_enqueue_tail() such that HTB can use it
instead.

The only other caller of __qdisc_enqueue_tail() is
qdisc_enqueue_tail() so we can move the backlog and return value
handling (which HTB doesn't need/want) to the latter.

Signed-off-by: David S. Miller <davem@davemloft.net>