KEYS: Permit in-place link replacement in keyring list

Make use of the previous patch that makes the garbage collector perform RCU
synchronisation before destroying defunct keys.  Key pointers can now be
replaced in-place without creating a new keyring payload and replacing the
whole thing as the discarded keys will not be destroyed until all currently
held RCU read locks are released.

If the keyring payload space needs to be expanded or contracted, then a
replacement will still need allocating, and the original will still have to be
freed by RCU.

Signed-off-by: David Howells <dhowells@redhat.com>

include/keys/keyring-type.h

@@ -24,7 +24,7 @@ struct keyring_list {
 	unsigned short	maxkeys;	/* max keys this list can hold */
 	unsigned short	nkeys;		/* number of keys currently held */
 	unsigned short	delkey;		/* key to be unlinked by RCU */
-	struct key	*keys[0];
+	struct key __rcu *keys[0];
 };