bird2: document the OSPF alignment patch

The patch carried only a subject line and a placeholder commit id, so
nothing recorded why marking struct top_hash_entry as PACKED fixes a
bus error, or whether it can be dropped.

Restore the description and the sign-off from the commit that
introduced it in the routing feed, keeping the original author's
wording, and record the upstream status. Verified that BIRD 2.19.2
still declares the structure without any alignment attribute, so the
patch is still needed.

The header keeps the "---" separator the sibling patch uses, and
orders Signed-off-by last among the trailers.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

net/bird2/patches/000-ospf-bus-error.patch

@@ -3,8 +3,24 @@ From: Matt Reeve <matt@mreeve.com>
 Date: Tue, 10 Aug 2021 12:41:04 +0100
 Subject: [PATCH] Fix bus error on OSPF on IPQ806X
 
----
+On a Netgear R7800, if ospf v2 or v3 is configured in bird.conf, it fails
+to start with this error:
 
+    kernel: Alignment trap: not handling instruction f44c0a1f at [<00035848>]
+    kernel: Unhandled fault: alignment exception (0x801) at 0x007e0624
+
+The problem is due to a struct not being properly aligned on the ARMv7
+architecture.
+
+This patch fixes the problem by adding the "PACKED" macro to the affected
+struct. Note that upstream may later fix this in another way, in which
+case this patch will not be required.
+
+Upstream-Status: not submitted. As of 2.19.2 the structure still carries
+no alignment attribute.
+
+Signed-off-by: Matt Reeve <matt@mreeve.com>
+---
 --- a/proto/ospf/topology.h
 +++ b/proto/ospf/topology.h
 @@ -41,7 +41,7 @@ struct top_hash_entry