mm/hugetlb: add missing VM_FAULT_SET_HINDEX in hugetlb_wp

commit 1cb9dc4b

 ("mm: hwpoison: support recovery from HugePage
copy-on-write faults") added support to use the mc variants when coping
hugetlb pages on CoW faults.

Add the missing VM_FAULT_SET_HINDEX, so the right si_addr_lsb will be
passed to userspace to report the extension of the faulty area.

Link: https://lkml.kernel.org/r/20240509100148.22384-3-osalvador@suse.de
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Axel Rasmussen <axelrasmussen@google.com>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>