ethtool: Validate module EEPROM offset as part of policy

Validate the offset to read from module EEPROM as part of the netlink
policy and remove the corresponding check from the code.

This also makes it possible to query the offset range from user space:

 $ genl ctrl policy name ethtool
 ...
 ID: 0x14  policy[32]:attr[2]: type=U32 range:[0,255]
 ...
Signed-off-by: NIdo Schimmel <idosch@nvidia.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ethtool/eeprom.c

@@ -177,10 +177,6 @@ static int eeprom_parse_request(struct ethnl_req_info *req_info, struct nlattr *
 		NL_SET_ERR_MSG_ATTR(extack, tb[ETHTOOL_A_MODULE_EEPROM_LENGTH],
 				    "reading cross half page boundary is illegal");
 		return -EINVAL;
-	} else if (request->offset >= ETH_MODULE_EEPROM_PAGE_LEN * 2) {
-		NL_SET_ERR_MSG_ATTR(extack, tb[ETHTOOL_A_MODULE_EEPROM_OFFSET],
-				    "offset is out of bounds");
-		return -EINVAL;
 	} else if (request->offset + request->length > ETH_MODULE_EEPROM_PAGE_LEN * 2) {
 		NL_SET_ERR_MSG_ATTR(extack, tb[ETHTOOL_A_MODULE_EEPROM_LENGTH],
 				    "reading cross page boundary is illegal");
@@ -233,7 +229,8 @@ const struct ethnl_request_ops ethnl_module_eeprom_request_ops = {
 
 const struct nla_policy ethnl_module_eeprom_get_policy[] = {
 	[ETHTOOL_A_MODULE_EEPROM_HEADER]	= NLA_POLICY_NESTED(ethnl_header_policy),
-	[ETHTOOL_A_MODULE_EEPROM_OFFSET]	= { .type = NLA_U32 },
+	[ETHTOOL_A_MODULE_EEPROM_OFFSET]	=
+		NLA_POLICY_MAX(NLA_U32, ETH_MODULE_EEPROM_PAGE_LEN * 2 - 1),
 	[ETHTOOL_A_MODULE_EEPROM_LENGTH]	=
 		NLA_POLICY_RANGE(NLA_U32, 1, ETH_MODULE_EEPROM_PAGE_LEN),
 	[ETHTOOL_A_MODULE_EEPROM_PAGE]		= { .type = NLA_U8 },