MAINTAINERS: fix regex pattern in CORE MM category

The pattern "include/linux/page[-_]*" matches every file that starts with
"page", because it's a regex and not a glob (so it has the meaning of
include/linux/page + match [-_] 0+ times).

Fix it up into a more regex-correct expression. Doing so reduces CC's
drastically in patches that touch pagemap.h (which is maintained as part
of PAGE CACHE).

As a side-effect, move linux/pageblock-flags.h explicitly under PAGE
ALLOCATOR.

Link: https://lore.kernel.org/linux-mm/20260422005608.342028-1-fmayle@google.com/
Link: https://lore.kernel.org/20260422123726.517220-1-pfalcato@suse.de
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>