In single-queue (block layer) mode,the function null_rq_prep_fn stops
the device if alloc_cmd fails. Then, once stopped, the device must be
restarted on the next command completion, so that the request(s) for
which alloc_cmd failed can be requeued. Otherwise the device hangs.

Unfortunately, device restart is currently performed only for delayed
completions, i.e., in irqmode==2. This fact causes hangs, for the
above reasons, with the other irqmodes in combination with single-queue
block layer.

This commits addresses this issue by making sure that, if stopped, the
device is properly restarted for all irqmodes on completions.
Signed-off-by: NPaolo Valente <paolo.valente@unimore.it>
Signed-off-by: NArianna AVanzini <avanzini@google.com>
Signed-off-by: NJens Axboe <axboe@fb.com>