net_sched: fix THROTTLED/RUNNING race

commit fd245a4a (net_sched: move TCQ_F_THROTTLED flag)
added a race.

qdisc_watchdog() is run from softirq, so special care should be taken or
we can lose one state transition (THROTTLED/RUNNING)

Prior to fd245a4a, we were manipulating q->flags (qdisc->flags &=
~TCQ_F_THROTTLED;) and this manipulation could only race with
qdisc_warn_nonwc().

Since we want to avoid atomic ops in qdisc fast path - it was the
meaning of commit 37112105 (QDISC_STATE_RUNNING dont need atomic
bit ops) - fix is to move THROTTLE bit into 'state' field, this one
being manipulated with SMP and IRQ safe operations.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>