rxrpc: Create an address for sendmsg() to bind unbound socket with

Create an address for sendmsg() to bind unbound socket with rather than
using a completely blank address otherwise the transport socket creation
will fail because it will try to use address family 0.

We use the address family specified in the protocol argument when the
AF_RXRPC socket was created and SOCK_DGRAM as the default.  For anything
else, bind() must be used.
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/af_rxrpc.c

@@ -401,6 +401,18 @@ static int rxrpc_sendmsg(struct socket *sock, struct msghdr *m, size_t len)
 
 	switch (rx->sk.sk_state) {
 	case RXRPC_UNBOUND:
+		rx->srx.srx_family = AF_RXRPC;
+		rx->srx.srx_service = 0;
+		rx->srx.transport_type = SOCK_DGRAM;
+		rx->srx.transport.family = rx->family;
+		switch (rx->family) {
+		case AF_INET:
+			rx->srx.transport_len = sizeof(struct sockaddr_in);
+			break;
+		default:
+			ret = -EAFNOSUPPORT;
+			goto error_unlock;
+		}
 		local = rxrpc_lookup_local(&rx->srx);
 		if (IS_ERR(local)) {
 			ret = PTR_ERR(local);