MAINTAINERS: add mm GUP section

As part of the ongoing efforts to sub-divide memory management
maintainership and reviewership, establish a section for GUP (Get User
Pages) support and add appropriate maintainers and reviewers.

Link: https://lkml.kernel.org/r/20250506173601.97562-1-lorenzo.stoakes@oracle.com


Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>