server: sanity check incoming command size (and payload)

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/server.h b/server.h
index 3a279f0..2958e73 100644
--- a/server.h
+++ b/server.h
@@ -41,6 +41,7 @@
 	FIO_SERVER_VER			= 33,
 
 	FIO_SERVER_MAX_FRAGMENT_PDU	= 1024,
+	FIO_SERVER_MAX_CMD_MB		= 2048,
 
 	FIO_NET_CMD_QUIT		= 1,
 	FIO_NET_CMD_EXIT		= 2,