mm: drop fake head checks

With fake head pages eliminated in the previous commit, remove the
supporting infrastructure:

  - page_fixed_fake_head(): no longer needed to detect fake heads;
  - page_is_fake_head(): no longer needed;
  - page_count_writable(): no longer needed for RCU protection;
  - RCU read_lock in page_ref_add_unless(): no longer needed;

This substantially simplifies compound_head() and page_ref_add_unless(),
removing both branches and RCU overhead from these hot paths.

RCU was required to serialize allocation of hugetlb page against
get_page_unless_zero() and prevent writing to read-only fake head.  It is
redundant without fake heads.

See bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN
walkers") for more details.

synchronize_rcu() in mm/hugetlb_vmemmap.c will be removed by a separate
patch.

Link: https://lkml.kernel.org/r/20260227194302.274384-14-kas@kernel.org
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: WANG Xuerui <kernel@xen0n.name>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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