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>