MAINTAINERS: add mm-related procfs files to MM sections

MAINTAINERS: add mm-related procfs files to MM sections

Some procfs files are very much related to memory management so let's have
MAINTAINERS reflect that.

Add fs/proc/meminfo.c to MEMORY MANAGEMENT - CORE.

Add fs/proc/task_[no]mmu.c to MEMORY MAPPING.

Link: https://lkml.kernel.org/r/20260305-maintainers-proc-v1-1-d6d09b3db3b6@kernel.org
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Acked-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>