Commit c347a787 (drbd: set ->bi_bdev in drbd_req_new) moved a
bio_set_dev call (which has since been removed) to "earlier", from
drbd_request_prepare to drbd_req_new.

The problem is that this accesses device->ldev->backing_bdev, which is
not NULL-checked at this point. When we don't have an ldev (i.e. when
the DRBD device is diskless), this leads to a null pointer deref.

So, only allocate the private_bio if we actually have a disk. This is
also a small optimization, since we don't clone the bio to only to
immediately free it again in the diskless case.

Fixes: c347a787 ("drbd: set ->bi_bdev in drbd_req_new")
Co-developed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Co-developed-by: Joel Colledge <joel.colledge@linbit.com>
Signed-off-by: Joel Colledge <joel.colledge@linbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221020085205.129090-1-christoph.boehmwalder@linbit.comSigned-off-by: Jens Axboe <axboe@kernel.dk>