MAINTAINERS: add core mm section

MAINTAINERS: add core mm section

In furtherance of ongoing efforts to ensure people are aware of who
de-facto maintains/has an interest in specific parts of mm, as well trying
to avoid get_maintainers.pl listing only Andrew and the mailing list for
mm files - establish a 'core' memory management section establishing David
as co-maintainer alongside Andrew (thanks David for volunteering!) along
with a number of relevant reviewers.

We try to keep things as fine-grained as possible, so we place only
obviously 'general' mm things here.  For files which are specific to a
particular part of mm, we prefer new entries.

Link: https://lkml.kernel.org/r/20250423123042.59082-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>