KEYS: Split role of the keyring pointer for keyring restrict functions

The first argument to the restrict_link_func_t functions was a keyring
pointer. These functions are called by the key subsystem with this
argument set to the destination keyring, but restrict_link_by_signature
expects a pointer to the relevant trusted keyring.

Restrict functions may need something other than a single struct key
pointer to allow or reject key linkage, so the data used to make that
decision (such as the trust keyring) is moved to a new, fourth
argument. The first argument is now always the destination keyring.

Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>

include/keys/system_keyring.h

@@ -18,7 +18,8 @@
 
 extern int restrict_link_by_builtin_trusted(struct key *keyring,
 					    const struct key_type *type,
-					    const union key_payload *payload);
+					    const union key_payload *payload,
+					    struct key *restriction_key);
 
 #else
 #define restrict_link_by_builtin_trusted restrict_link_reject
@@ -28,7 +29,8 @@ extern int restrict_link_by_builtin_trusted(struct key *keyring,
 extern int restrict_link_by_builtin_and_secondary_trusted(
 	struct key *keyring,
 	const struct key_type *type,
-	const union key_payload *payload);
+	const union key_payload *payload,
+	struct key *restriction_key);
 #else
 #define restrict_link_by_builtin_and_secondary_trusted restrict_link_by_builtin_trusted
 #endif