iprule: amend ipproto netlink nla_put_u32 to nla_put_u8

iprule: amend ipproto netlink nla_put_u32 to nla_put_u8

FRA_IP_PROTO expects an 8 bit value.

Follow-up fix for d29cf707478cfc5465fc8a7b8ccfde72a739a4f6.

uint8 prevents the kernel log message:
netlink: 'netifd': attribute type 22 has an invalid length.

The message is otherwise harmless; ip rules using ipproto are created
successfully.

Tested on 24.10-snapshot

Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/netifd/pull/42
Signed-off-by: Robert Marko <[email protected]>