Merge branch 'vsock-virtio-fix-data-loss-disclosure-due-to-joining-of-non-linear...

Michal Luczaj says:

====================
vsock/virtio: Fix data loss/disclosure due to joining of non-linear skb

Loopback transport coalesces some skbs too eagerly. Handling a zerocopy
(non-linear) skb as a linear one leads to skb data loss and kernel memory
disclosure.

Plug the loss/leak by allowing only linear skb join. Provide a test.
====================

Link: https://patch.msgid.link/20260113-vsock-recv-coalescence-v2-0-552b17837cf4@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>