Merge branch 'preserve-msg_zerocopy-with-forwarding'

Willem de Bruijn says:

====================
preserve MSG_ZEROCOPY with forwarding

Avoid false positive copying of zerocopy skb frags when entering the
ingress path if the skb is not queued locally but forwarded.

Patch 1 for more details and feature.

Patch 2 converts the existing selftest to a pass/fail test and adds
        coverage for this new feature.
====================

Link: https://patch.msgid.link/20250630194312.1571410-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>