null_blk: change configfs dependency to select

A recent commit made null_blk depend on configfs, which is kind of
annoying since you now have to find this dependency and enable that
as well. Discovered this since I no longer had null_blk available
on a box I needed to debug, since it got killed when the config
updated after the configfs change was merged.

Fixes: 3bf2bd20 ("nullb: add configfs interface")
Reviewed-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>