keys: Provide request_key_rcu()

Provide a request_key_rcu() function that can be used to request a key
under RCU conditions.  It can only search and check permissions; it cannot
allocate a new key, upcall or wait for an upcall to complete.  It may
return a partially constructed key.

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

include/linux/key.h

@@ -274,6 +274,9 @@ extern struct key *request_key(struct key_type *type,
 			       const char *description,
 			       const char *callout_info);
 
+extern struct key *request_key_rcu(struct key_type *type,
+				   const char *description);
+
 extern struct key *request_key_with_auxdata(struct key_type *type,
 					    const char *description,
 					    const void *callout_info,