lightnvm: pblk: control I/O flow also on tear down

When removing a pblk instance, control the write I/O flow to the
controller as we do in the fast path.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>