mm: clarify the folio_free_swap() for do_swap_page()

Since commit 4b34f1d82c654 ("mm, swap: free the swap cache after folio is
mapped"), we have relied on do_wp_page() to handle the non-exclusive case,
where the folio may either be reused or require CoW.

As a result, using the refcount in do_swap_page() to decide when to free
the swap cache is no longer necessary, since do_wp_page() can handle this
more cleanly and consistently.

We can now simply use FAULT_FLAG_WRITE together with exclusivity to decide
when to free the swap cache in do_swap_page().

Link: https://lore.kernel.org/20260701235955.36126-5-baohua@kernel.org
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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