MAINTAINERS: Update maintainer entry for NBD

The old maintainers email is bouncing and I've rewritten most of this
driver in the recent months.  Also add linux-block to the mailinglist
and remove the old tree, I will send patches through the linux-block
tree.  Thanks,

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