um-ubd: Entrust re-queue to the upper layers

Fixes crashes due to ubd requeue logic conflicting with the block-mq
logic. Crash is reproducible in 5.0 - 5.3.

Fixes: 53766def

 ("um: Clean-up command processing in UML UBD driver")
Cc: stable@vger.kernel.org # v5.0+
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>