MAINTAINERS: add memory advice section

MAINTAINERS: add memory advice section

The madvise code straddles both VMA and page table manipulation.  As a
result, separate it out into its own section and add maintainers/reviewers
as appropriate.

We additionally include the mman-common.h file as this contains the shared
madvise flags and it is important we maintain this alongside madvise.c.

Link: https://lkml.kernel.org/r/20250411072724.10841-1-lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Jann Horn <jannh@google.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>