fscache_cookie_enabled: check cookie is valid before accessing it

fscache_cookie_enabled() could be called on NULL cookies and cause a
null pointer dereference when accessing cookie flags: just make sure
the cookie is valid first

Suggested-by: David Howells <dhowells@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

include/linux/fscache.h

@@ -167,7 +167,7 @@ struct fscache_cookie {
 
 static inline bool fscache_cookie_enabled(struct fscache_cookie *cookie)
 {
-	return test_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags);
+	return fscache_cookie_valid(cookie) && test_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags);
 }
 
 /*