rxrpc: Fix server keyring leak

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: 17926a79 ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/key.c

@@ -903,7 +903,7 @@ int rxrpc_request_key(struct rxrpc_sock *rx, sockptr_t optval, int optlen)
 
 	_enter("");
 
-	if (optlen <= 0 || optlen > PAGE_SIZE - 1)
+	if (optlen <= 0 || optlen > PAGE_SIZE - 1 || rx->securities)
 		return -EINVAL;
 
 	description = memdup_sockptr_nul(optval, optlen);