ethtool: Validate module EEPROM length as part of policy

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

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

 $ genl ctrl policy name ethtool
 ...
 ID: 0x14  policy[32]:attr[3]: type=U32 range:[1,128]
 ...
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethtool/eeprom.c

@@ -159,9 +159,6 @@ static int eeprom_parse_request(struct ethnl_req_info *req_info, struct nlattr *
 	request->offset = nla_get_u32(tb[ETHTOOL_A_MODULE_EEPROM_OFFSET]);
 	request->length = nla_get_u32(tb[ETHTOOL_A_MODULE_EEPROM_LENGTH]);
 
-	if (!request->length)
-		return -EINVAL;
-
 	/* The following set of conditions limit the API to only dump 1/2
 	 * EEPROM page without crossing low page boundary located at offset 128.
 	 * This means user may only request dumps of length limited to 128 from
@@ -237,7 +234,8 @@ 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_LENGTH]	= { .type = NLA_U32 },
+	[ETHTOOL_A_MODULE_EEPROM_LENGTH]	=
+		NLA_POLICY_RANGE(NLA_U32, 1, ETH_MODULE_EEPROM_PAGE_LEN),
 	[ETHTOOL_A_MODULE_EEPROM_PAGE]		= { .type = NLA_U8 },
 	[ETHTOOL_A_MODULE_EEPROM_BANK]		= { .type = NLA_U8 },
 	[ETHTOOL_A_MODULE_EEPROM_I2C_ADDRESS]	=