MAINTAINERS: update list for NBD

nbd-general@sourceforge.net becomes nbd@other.debian.org, because
sourceforge is just a spamtrap these days.

Signed-off-by: Wouter Verhelst <w@uter.be>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>