macvlan: fix possible UAF in macvlan_forward_source()

These records connect through source-stated identifiers. Shared identifiers or dates alone do not establish shared exposure, root cause or applicability.

CVE-2026-23001 · Microsoft / collected KEV context

cve-program · CVE-2026-23001

• https://git.kernel.org/stable/c/8133e85b8a3ec9f10d861e0002ec6037256e987e
• https://git.kernel.org/stable/c/484919832e2db6ce1e8add92c469e5d459a516b5
• https://git.kernel.org/stable/c/232afc74a6dde0fe1830988e5827921f5ec9bb3f
• https://git.kernel.org/stable/c/15f6faf36e162532bec5cc05eb3fc622108bf2ed
• https://git.kernel.org/stable/c/8518712a2ca952d6da2238c6f0a16b4ae5ea3f13
• https://git.kernel.org/stable/c/6dbead9c7677186f22b7981dd085a0feec1f038e
• https://git.kernel.org/stable/c/7470a7a63dc162f07c26dbf960e41ee1e248d80e

No related operational record is retained in the current change model.