fscrypt: Add some folio helper functions

fscrypt_is_bounce_folio() is the equivalent of fscrypt_is_bounce_page()
and fscrypt_pagecache_folio() is the equivalent of fscrypt_pagecache_page().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20230324180129.1220691-3-willy@infradead.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

include/linux/fscrypt.h

@@ -273,6 +273,16 @@ static inline struct page *fscrypt_pagecache_page(struct page *bounce_page)
 	return (struct page *)page_private(bounce_page);
 }
 
+static inline bool fscrypt_is_bounce_folio(struct folio *folio)
+{
+	return folio->mapping == NULL;
+}
+
+static inline struct folio *fscrypt_pagecache_folio(struct folio *bounce_folio)
+{
+	return bounce_folio->private;
+}
+
 void fscrypt_free_bounce_page(struct page *bounce_page);
 
 /* policy.c */
@@ -445,6 +455,17 @@ static inline struct page *fscrypt_pagecache_page(struct page *bounce_page)
 	return ERR_PTR(-EINVAL);
 }
 
+static inline bool fscrypt_is_bounce_folio(struct folio *folio)
+{
+	return false;
+}
+
+static inline struct folio *fscrypt_pagecache_folio(struct folio *bounce_folio)
+{
+	WARN_ON_ONCE(1);
+	return ERR_PTR(-EINVAL);
+}
+
 static inline void fscrypt_free_bounce_page(struct page *bounce_page)
 {
 }