MAINTAINERS: add entry for block io cgroup

This links mailing list cgroups@vger.kernel.org with related files.

$ ./scripts/get_maintainer.pl -f block/blk-cgroup.c
Jens Axboe <axboe@kernel.dk> (maintainer:BLOCK LAYER)
cgroups@vger.kernel.org (open list:CONTROL GROUP - BLOCK IO CONTROLLER (BLKIO))
linux-block@vger.kernel.org (open list:BLOCK LAYER)
linux-kernel@vger.kernel.org (open list)

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

Added git tree/maintainer entries from Tejun.

Signed-off-by: Jens Axboe <axboe@kernel.dk>