ceph: add bug tracking system info to MAINTAINERS

This patch adds information about Ceph bug tracking system.

[ idryomov: add the same for RBD, don't mention include/linux/ceph/
  again ]

Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>