samples/bpf: deal with EBUSY return code from sendmsg in xdpsock sample

Sendmsg in the SKB path of AF_XDP can now return EBUSY when a packet
was discarded and completed by the driver. Just ignore this message
in the sample application.

Fixes: b4b8faa1 ("samples/bpf: sample application and documentation for AF_XDP sockets")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Reported-by: Pavel Odintsov <pavel@fastnetmon.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

samples/bpf/xdpsock_user.c

@@ -729,7 +729,7 @@ static void kick_tx(int fd)
 	int ret;
 
 	ret = sendto(fd, NULL, 0, MSG_DONTWAIT, NULL, 0);
-	if (ret >= 0 || errno == ENOBUFS || errno == EAGAIN)
+	if (ret >= 0 || errno == ENOBUFS || errno == EAGAIN || errno == EBUSY)
 		return;
 	lassert(0);
 }