RDMA: Add Jason Gunthorpe as a co-maintainer

As was discussed in September and October, add Jason along with
Doug to have a team maintainership model for the RDMA subystem.

Mellanox Technologies will be funding Jason's independent work on
the maintainership.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>