null_blk: remove NULLB_DEV_FL_CONFIGURED on turning off nullb device

Currently mbps knob could only be set once before switching power knob to
on, after power knob has been set at least once, there is no way to set
mbps knob again due to -EBUSY.

As nullb is mainly used for testing, in order to make it flexible, this
removes the flag NULLB_DEV_FL_CONFIGURED so that mbps knob can be reset
when power knob is off, e.g.

echo 0 > /config/nullb/a/power
echo 40 > /config/nullb/a/mbps
echo 1 > /config/nullb/a/power

So does other knobs under /config/nullb/a.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>