netlink: add nl_set_extack_cookie_u32()

Similar to existing nl_set_extack_cookie_u64(), add new helper
nl_set_extack_cookie_u32() which sets extack cookie to a u32 value.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>