mm: update MAINTAINERS entry for HMM

Jérôme has moved on from RH and has not been looking at HMM patches for
some time. I've made the most changes to the core code in the recent
period and Leon is now working on the HMM side from the RDMA ODP.

Link: https://lkml.kernel.org/r/0-v1-a1df5219c7a3+1d981-hmm_maintainers_jgg@nvidia.com
Closes: https://lore.kernel.org/all/39d43309-9f34-48bc-a9ad-108c607ba175@samsung.com/


Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Leon Romanovsky <leon@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>