aoe: avoid running request handler on plugged queue

Calling the request handler directly on a plugged queue defeats
the performance improvements provided by the plugging mechanism.
Use the __blk_run_queue function instead of calling the request
handler directly, so that we don't interfere with the block
layer's ability to plug the queue.

Signed-off-by: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>