MAINTAINERS: add secondary tree for ceph modules

The Ceph kernel code is primarily developed in the github tree, and only
pushed to the korg tree before going to Linus.  If Sage is unavailable and
another maintainer needs to push something upstream, pull requests may
originate from the github tree instead of Sage's korg tree.

Signed-off-by: Sage Weil <sage@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>