userfaultfd: introduce vm_uffd_ops->alloc_folio()

and use it to refactor mfill_atomic_pte_zeroed_folio() and
mfill_atomic_pte_copy().

mfill_atomic_pte_zeroed_folio() and mfill_atomic_pte_copy() perform
almost identical actions:
* allocate a folio
* update folio contents (either copy from userspace of fill with zeros)
* update page tables with the new folio

Split a __mfill_atomic_pte() helper that handles both cases and uses newly
introduced vm_uffd_ops->alloc_folio() to allocate the folio.

Pass the ops structure from the callers to __mfill_atomic_pte() to later
allow using anon_uffd_ops for MAP_PRIVATE mappings of file-backed VMAs.

Note, that the new ops method is called alloc_folio() rather than
folio_alloc() to avoid clash with alloc_tag macro folio_alloc().

Link: https://lore.kernel.org/20260402041156.1377214-10-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: James Houghton <jthoughton@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrei Vagin <avagin@google.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Harry Yoo (Oracle) <harry@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nikita Kalyazin <kalyazin@amazon.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Carlier <devnexen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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