mm/huge_memory: separate out CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic

mm/huge_memory: separate out CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic

Rather than mixing the refcounted and non-refcounted
CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic, separate the two out cleanly so
it is clear what happens when this configuration option is set and what
happens when it is not.

Introduce HUGE_ZERO_UNSET_PFN to abstract the ~0UL assignment, only
introduce the refcount, lock and shrinker if
!CONFIG_PERSISTENT_HUGE_ZERO_FOLIO, abstract initialisation and teardown,
abstract the huge zero folio allocation from refcounting.

Also change a BUG_ON() to WARN_ON_ONCE() while we're at it.

No functional change intended.

Link: https://lore.kernel.org/20260730-fix-refcounted-huge-zero-v2-2-c5d8a41b317f@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Fixes: 3b77e8c8cde5 ("mm/thp: make is_huge_zero_pmd() safe and quicker")
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Hengbin Zhang <uqbarz@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>