KEYS: Permit key_serial() to be called with a const key pointer

Permit key_serial() to be called with a const key pointer.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

include/linux/key.h

@@ -271,7 +271,7 @@ extern int keyring_add_key(struct key *keyring,
 
 extern struct key *key_lookup(key_serial_t id);
 
-static inline key_serial_t key_serial(struct key *key)
+static inline key_serial_t key_serial(const struct key *key)
 {
 	return key ? key->serial : 0;
 }