bonding: print churn state via netlink

Currently, the churn state is printed only in sysfs. Add netlink support
so users could get the state via netlink.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20260224020215.6012-1-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/uapi/linux/if_link.h

@@ -1568,6 +1568,8 @@ enum {
 	IFLA_BOND_SLAVE_AD_PARTNER_OPER_PORT_STATE,
 	IFLA_BOND_SLAVE_PRIO,
 	IFLA_BOND_SLAVE_ACTOR_PORT_PRIO,
+	IFLA_BOND_SLAVE_AD_CHURN_ACTOR_STATE,
+	IFLA_BOND_SLAVE_AD_CHURN_PARTNER_STATE,
 	__IFLA_BOND_SLAVE_MAX,
 };