mm/hugetlb: rename avoid_reserve to cow_from_owner

mm/hugetlb: rename avoid_reserve to cow_from_owner

The old name "avoid_reserve" can be too generic and can be used wrongly in
the new call sites that want to allocate a hugetlb folio.

It's confusing on two things: (1) whether one can opt-in to avoid global
reservation, and (2) whether it should take more than one count.

In reality, this flag is only used in an extremely hacky path, in an
extremely hacky way in hugetlb CoW path only, and always use with 1 saying
"skip global reservation".  Rename the flag to avoid future abuse of this
flag, making it a boolean so as to reflect its true representation that
it's not a counter.  To make it even harder to abuse, add a comment above
the function to explain it.

Link: https://lkml.kernel.org/r/20250107204002.2683356-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
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: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>