MAINTAINERS: add Barry as a THP reviewer

I have been actively contributing to mTHP and reviewing related patches
for an extended period, and I would like to continue supporting patch
reviews.

Link: https://lkml.kernel.org/r/20250609002442.1856-1-21cnbao@gmail.com
Signed-off-by: Barry Song <baohua@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Dev Jain <dev.jain@arm.com>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Barry Song

MAINTAINERS

@@ -15919,6 +15919,7 @@ R:	Liam R. Howlett <Liam.Howlett@oracle.com>
 R:	Nico Pache <npache@redhat.com>
 R:	Ryan Roberts <ryan.roberts@arm.com>
 R:	Dev Jain <dev.jain@arm.com>
+R:	Barry Song <baohua@kernel.org>
 L:	linux-mm@kvack.org
 S:	Maintained
 W:	http://www.linux-mm.org