If someone calls setsockopt() twice to set a server key keyring, the first
keyring is leaked.
Fix it to return an error instead if the server key keyring is already set.
Fixes: 17926a7932 ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
Signed-off-by: David Howells <dhowells@redhat.com>