block: only force kblockd unplugging from the schedule() path

For the explicit unplugging, we'd prefer to kick things off
immediately and not pay the penalty of the latency to switch
to kblockd. So let blk_finish_plug() do the run inline, while
the implicit-on-schedule-out unplug will punt to kblockd.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>