cxgb4: add m4 check for net_device.max_mtu

If max_mtu is part of struct net_device, then define
HAVE_NET_DEVICE_MAX_MTU.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>

config/rdma.m4

@@ -4729,6 +4729,24 @@ AC_DEFUN([LINUX_CONFIG_COMPAT],
 	],[
 		AC_MSG_RESULT(no)
 	])
+
+	AC_MSG_CHECKING([if struct net_device has max_mtu])
+	LB_LINUX_TRY_COMPILE([
+		#include <linux/netdevice.h>
+	],[
+		struct net_device *dev = NULL;
+
+		dev->max_mtu = 0;
+
+		return 0;
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_NET_DEVICE_MAX_MTU, 1,
+			  [max_mtu is in struct net_device])
+	],[
+		AC_MSG_RESULT(no)
+	])
+
 ])
 #
 # COMPAT_CONFIG_HEADERS