mm/hugetlb: unify restore reserve accounting for new allocations

Either hugetlb pages dequeued from hstate, or newly allocated from buddy,
would require restore-reserve accounting to be managed properly.  Merge
the two paths on it.  Add a small comment to make it slightly nicer.

Link: https://lkml.kernel.org/r/20250107204002.2683356-8-peterx@redhat.com


Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Ackerley Tng <ackerleytng@google.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>