Merge branch 'af_unix-fix-inconsistent-scc_index'

Kuniyuki Iwashima says:

====================
af_unix: Fix inconsistent scc_index.

James Burton reported that a single SCC could have multiple
scc_index and unix_vertex_dead() fails to detect a dead SCC.

Patch 1 fixes it and Patch 2 adds a test case.
====================

Link: https://patch.msgid.link/20260912030852.1467872-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>