mm: hugetlb_cma: mark hugetlb_cma{_only} as __ro_after_init

mm: hugetlb_cma: mark hugetlb_cma{_only} as __ro_after_init

hugetlb_cma and hugetlb_cma_only are initialized once during init and
never changed.

Link: https://lkml.kernel.org/r/20260112150954.1802953-6-wangkefeng.wang@huawei.com
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>