cacheflush.h: Add forward declaration for struct folio

The struct folio is not declared in cacheflush.h so we need to provide
a forward declaration as otherwise users of this header file may get
warnings.
Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 522a0032 ("Add linux/cacheflush.h")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cacheflush.h

@@ -4,6 +4,8 @@
 
 #include <asm/cacheflush.h>
 
+struct folio;
+
 #if ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE
 #ifndef ARCH_IMPLEMENTS_FLUSH_DCACHE_FOLIO
 void flush_dcache_folio(struct folio *folio);