ethtool: Fix NULL pointer dereference during module EEPROM dump

When get_module_eeprom_by_page() is not implemented by the driver, NULL
pointer dereference can occur [1].

Fix by testing if get_module_eeprom_by_page() is implemented instead of
get_module_info().

[1]
 BUG: kernel NULL pointer dereference, address: 0000000000000000
 [...]
 CPU: 0 PID: 251 Comm: ethtool Not tainted 5.13.0-rc3-custom-00940-g3822d067 #989
 Call Trace:
  eeprom_prepare_data+0x101/0x2d0
  ethnl_default_doit+0xc2/0x290
  genl_family_rcv_msg_doit+0xdc/0x140
  genl_rcv_msg+0xd7/0x1d0
  netlink_rcv_skb+0x49/0xf0
  genl_rcv+0x1f/0x30
  netlink_unicast+0x1f6/0x2c0
  netlink_sendmsg+0x1f9/0x400
  __sys_sendto+0xe1/0x130
  __x64_sys_sendto+0x1b/0x20
  do_syscall_64+0x3a/0x70
  entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: c97a31f6 ("ethtool: wire in generic SFP module access")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethtool/eeprom.c

@@ -95,7 +95,7 @@ static int get_module_eeprom_by_page(struct net_device *dev,
 	if (dev->sfp_bus)
 		return sfp_get_module_eeprom_by_page(dev->sfp_bus, page_data, extack);
 
-	if (ops->get_module_info)
+	if (ops->get_module_eeprom_by_page)
 		return ops->get_module_eeprom_by_page(dev, page_data, extack);
 
 	return -EOPNOTSUPP;