mm, swap: remove no longer used swap mapping helper

This helper existed to fix the circular header dependency issue but it is
no longer used since commit 0d40cfe ("fs: remove
folio_file_mapping()"), remove it.

Link: https://lkml.kernel.org/r/20250430181052.55698-7-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Chao Yu <chao@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Sterba <dsterba@suse.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Joanne Koong <joannelkoong@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Qu Wenruo <wqu@suse.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: ryncsn

include/linux/pagemap.h

@@ -533,7 +533,6 @@ static inline void filemap_nr_thps_dec(struct address_space *mapping)
 }
 
 struct address_space *folio_mapping(struct folio *);
-struct address_space *swapcache_mapping(struct folio *);
 
 /**
  * folio_flush_mapping - Find the file mapping this folio belongs to.

mm/swapfile.c

@@ -3662,15 +3662,6 @@ struct swap_info_struct *swp_swap_info(swp_entry_t entry)
 	return swap_type_to_swap_info(swp_type(entry));
 }
 
-/*
- * out-of-line methods to avoid include hell.
- */
-struct address_space *swapcache_mapping(struct folio *folio)
-{
-	return swp_swap_info(folio->swap)->swap_file->f_mapping;
-}
-EXPORT_SYMBOL_GPL(swapcache_mapping);
-
 /*
  * add_swap_count_continuation - called when a swap count is duplicated
  * beyond SWAP_MAP_MAX, it allocates a new page and links that to the entry's