kernel-pfkey: Clear receive buffer before sending request

Many of the messages sent by the kernel, including confirmations to our
requests, are sent as broadcasts to all PF_KEY sockets.  So if an
external tool is used to manage SAs/policies (e.g. unrelated to IPsec)
the receive buffer might be filled, resulting in errors like these:

  error sending to PF_KEY socket: No buffer space available

To avoid this, just clear the buffer before sending any message.

Fixes #3225.

src/libcharon/plugins/kernel_pfkey/kernel_pfkey_ipsec.c

@@ -1145,6 +1145,23 @@ static status_t pfkey_send_socket(private_kernel_pfkey_ipsec_t *this, int socket
 
 	this->mutex_pfkey->lock(this->mutex_pfkey);
 
+	/* the kernel may broadcast messages not related to our requests (e.g. when
+	 * managing SAs and policies via an external tool), so let's clear the
+	 * receive buffer so there is room for our request and its reply. */
+	while (TRUE)
+	{
+		len = recv(socket, buf, sizeof(buf), MSG_DONTWAIT);
+
+		if (len < 0)
+		{
+			if (errno == EINTR)
+			{	/* interrupted, try again */
+				continue;
+			}
+			break;
+		}
+	}
+
 	/* FIXME: our usage of sequence numbers is probably wrong. check RFC 2367,
 	 * in particular the behavior in response to an SADB_ACQUIRE. */
 	in->sadb_msg_seq = ++this->seq;