nbd: pass the nbd pointer for flags debugfs

We were passing in &nbd for the private data in debugfs_create_file() for the
flags entry.  We expect it to just be nbd, fix this so we get proper output from
this debugfs entry.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>