mm/hugetlb: drop vma_has_reserves()

mm/hugetlb: drop vma_has_reserves()

After the previous cleanup, vma_has_reserves() is mostly an empty helper
except that it says "use reserve count" is inverted meaning from "needs a
global reserve count", which is still true.

To avoid confusions on having two inverted ways to ask the same question,
always use the gbl_chg everywhere, and drop the function.

When at it, rename "chg" to "gbl_chg" in dequeue_hugetlb_folio_vma().  It
might be helpful for readers to see that the "chg" here is the global
reserve count, not the vma resv count.

Link: https://lkml.kernel.org/r/20250107204002.2683356-7-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>