[PATCH 09/11] cleancache: Use static_key instead of cleancache_ops and cleancache_enabled.

As ways to determine whether to allow certain functions to be called. This makes it easier to understand the code - the three functions that can be called by the filesystem irregardless whether a backend is set or not cleancache_init_fs, cleancache_init_shared_fs, and cleancache_invalidate_fs.

[PATCH 09/11] cleancache: Use static_key instead of cleancache_ops and cleancache_enabled.

As ways to determine whether to allow certain functions to be called. This makes it easier to understand the code - the three functions that can be called by the filesystem irregardless whether a backend is set or not cleancache_init_fs, cleancache_init_shared_fs, and cleancache_invalidate_fs.