nexthop: initialize extack in nh_res_bucket_migrate()

• azl3 kernel 6.6.145.2-1 on Azure Linux 3.0

SecurityAlert records field-level changes from the point we begin following a bulletin. Earlier vendor changes may not have a field-by-field record.

1. Aug 20, 2026 at 7:00 PM UTC
   • The vendor changed its remediation guidance.

1. Aug 7, 2026 at 1:19 AM UTC
   Published by Microsoft Security Response Center

   The publication date reported by the vendor.

2. Aug 9, 2026 at 1:42 AM UTC
   Updated by Microsoft Security Response Center

   The vendor changed the advisory after it was first published.

3. Aug 20, 2026 at 6:50 PM UTC
   Added to SecurityAlert

   We collected the advisory from the official source.

4. Oct 28, 2026 at 3:45 AM UTC
   Confirmed at the source

   Our collector saw this advisory during a later source check.