netlink: specs: rt-route: add the route deletion reason

Add the del-reason attribute and its enum to the route attribute set,
and to the getroute reply, which the route notifications reuse.

The attribute is absent from the newroute and delroute request lists.
RTA_DEL_REASON is above strict_start_type in rtm_ipv6_policy, so
encoding it in a request is rejected with -EINVAL.

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
Link: https://patch.msgid.link/20260808005642.26901-10-sigefriedhyy@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/netlink/specs/rt-route.yaml

@@ -78,6 +78,27 @@ definitions:
       -
         name: rta-used
         type: u32
+  -
+    name: del-reason
+    type: enum
+    name-prefix: rt-del-reason-
+    enum-name: rt-del-reason
+    doc: |
+      Why the kernel deleted a route. New causes may be appended. The
+      value space is family-agnostic, a value is never reinterpreted
+      per address family.
+    entries:
+      -
+        name: unspec
+        doc: The deletion path does not record a cause.
+      -
+        name: expired
+        doc: The RTF_EXPIRES lifetime ran out and the route was garbage
+          collected.
+      -
+        name: ra-withdrawn
+        doc: A Router Advertisement withdrew the route with a zero
+          lifetime.
 
 attribute-sets:
   -
@@ -185,6 +206,18 @@ attribute-sets:
         type: u32
         byte-order: big-endian
         display-hint: hex
+      -
+        name: del-reason
+        type: u32
+        enum: del-reason
+        doc: |
+          Emitted only on RTM_DELROUTE notifications, and only when the
+          deletion path records a cause. An absent attribute means
+          either an older kernel or a deletion path that does not
+          record its cause, consumers must treat absent and unspec
+          identically. Currently only IPv6 deletion paths record a
+          cause. The attribute is notification-only, the kernel rejects
+          it in requests.
   -
     name: metrics
     name-prefix: rtax-
@@ -299,6 +332,7 @@ operations:
             - dport
             - nh-id
             - flowlabel
+            - del-reason
       dump:
         request:
           value: 26