server: serial is a 64-bit field

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/server.c b/server.c
index 44f5fa0..fe93ffb 100644
--- a/server.c
+++ b/server.c
@@ -162,7 +162,7 @@
 	struct fio_net_cmd cmd = {
 		.version	= cpu_to_le16(FIO_SERVER_VER1),
 		.opcode		= cpu_to_le16(opcode),
-		.serial		= cpu_to_le16(serial),
+		.serial		= cpu_to_le64(serial),
 	};
 
 	fio_net_cmd_crc(&cmd);