ceph: switch back to testing for NULL folio->private in ceph_dirty_folio

Willy requested that we change this back to warning on folio->private
being non-NULl. He's trying to kill off the PG_private flag, and so we'd
like to catch where it's non-NULL.

Add a VM_WARN_ON_FOLIO (since it doesn't exist yet) and change over to
using that instead of VM_BUG_ON_FOLIO along with testing the ->private
pointer.

[ xiubli: define VM_WARN_ON_FOLIO macro in case DEBUG_VM is disabled
reported by kernel test robot <[e-mail]> ]

Cc: Matthew Wilcox <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>
Signed-off-by: Xiubo Li <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

fs/ceph/addr.c

···
 	 * Reference snap context in folio->private.  Also set
 	 * PagePrivate so that we get invalidate_folio callback.
 	 */
-	VM_BUG_ON_FOLIO(folio_test_private(folio), folio);
+	VM_WARN_ON_FOLIO(folio->private, folio);
 	folio_attach_private(folio, snapc);
 
 	return ceph_fscache_dirty_folio(mapping, folio);

include/linux/mmdebug.h

···
 	}								\
 	unlikely(__ret_warn_once);					\
 })
+#define VM_WARN_ON_FOLIO(cond, folio)		({			\
+	int __ret_warn = !!(cond);					\
+									\
+	if (unlikely(__ret_warn)) {					\
+		dump_page(&folio->page, "VM_WARN_ON_FOLIO(" __stringify(cond)")");\
+		WARN_ON(1);						\
+	}								\
+	unlikely(__ret_warn);						\
+})
 #define VM_WARN_ON_ONCE_FOLIO(cond, folio)	({			\
 	static bool __section(".data.once") __warned;			\
 	int __ret_warn_once = !!(cond);					\
···
 #define VM_WARN_ON(cond) BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ON_ONCE_PAGE(cond, page)  BUILD_BUG_ON_INVALID(cond)
+#define VM_WARN_ON_FOLIO(cond, folio)  BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ON_ONCE_FOLIO(cond, folio)  BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN_ONCE(cond, format...) BUILD_BUG_ON_INVALID(cond)
 #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)