KEYS: Use keyring_alloc() to create special keyrings

Use keyring_alloc() to create special keyrings now that it has a permissions
parameter rather than using key_alloc() + key_instantiate_and_link().

Also document and export keyring_alloc() so that modules can use it too.

Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index cf704a9..8c25558 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -275,6 +275,7 @@
 
 	return keyring;
 }
+EXPORT_SYMBOL(keyring_alloc);
 
 /**
  * keyring_search_aux - Search a keyring tree for a key matching some criteria