MAINTAINERS: update maintainers for HugeTLB

Change my role to Maintainer as I am quite involved in HugeTLB
development, and will be more so with the upcoming HugetLB-pagewalk
unification, so I would like to help Munchun take care of the code. 
Besides, having two people will help in offloading some pressure.

Also add David as a Reviewer since he has quite some knowledge in the
field and has already provided valuable feedback.

Link: https://lkml.kernel.org/r/20250617185910.471406-1-osalvador@suse.de


Signed-off-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Muchun Song <muchun.song@linux.dev>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>