cleancache: Make cleancache_init use a pointer for the ops

Instead of using a backend_registered to determine whether a backend is
enabled.  This allows us to remove the backend_register check and just
do 'if (cleancache_ops)'

[v1: Rebase on top of b97c4b430b0a (ramster->zcache move]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Andor Daam <andor.daam@googlemail.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Florian Schmaus <fschmaus@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Stefan Hengelein <ilendir@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/cleancache.h b/include/linux/cleancache.h
index 42e55de..3af5ea8 100644
--- a/include/linux/cleancache.h
+++ b/include/linux/cleancache.h
@@ -33,7 +33,7 @@
 	void (*invalidate_fs)(int);
 };
 
-extern struct cleancache_ops
+extern struct cleancache_ops *
 	cleancache_register_ops(struct cleancache_ops *ops);
 extern void __cleancache_init_fs(struct super_block *);
 extern void __cleancache_init_shared_fs(char *, struct super_block *);