lightnvm: cleanup queue before target removal

This prevents outstanding IOs to be sent for completion to target after
the target has been removed. The flow is now: stop new IOs > cleanup
queue > remove target.

Signed-off-by: Javier Gonzalez <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>