rxrpc: The server keyring isn't network-namespaced

The keyring containing the server's tokens isn't network-namespaced, so it
shouldn't be looked up with a network namespace.  It is expected to be
owned specifically by the server, so namespacing is unnecessary.

Fixes: a58946c1 ("keys: Pass the network namespace into request_key mechanism")
Signed-off-by: David Howells <dhowells@redhat.com>