fs: remove folio_file_mapping()

No callers of this function remain as filesystems no longer see swapfile
pages through their normal read/write paths.

Link: https://lkml.kernel.org/r/20250217192009.437916-3-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Matthew Wilcox (Oracle)

include/linux/pagemap.h

@@ -535,26 +535,6 @@ 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_file_mapping - Find the mapping this folio belongs to.
- * @folio: The folio.
- *
- * For folios which are in the page cache, return the mapping that this
- * page belongs to.  Folios in the swap cache return the mapping of the
- * swap file or swap device where the data is stored.  This is different
- * from the mapping returned by folio_mapping().  The only reason to
- * use it is if, like NFS, you return 0 from ->activate_swapfile.
- *
- * Do not call this for folios which aren't in the page cache or swap cache.
- */
-static inline struct address_space *folio_file_mapping(struct folio *folio)
-{
-	if (unlikely(folio_test_swapcache(folio)))
-		return swapcache_mapping(folio);
-
-	return folio->mapping;
-}
-
 /**
  * folio_flush_mapping - Find the file mapping this folio belongs to.
  * @folio: The folio.