CVE-2026-74611: tls: rx: restore msg_iter before TLS 1.3 optimistic retry

In the Linux kernel, the following vulnerability has been resolved:

tls: rx: restore msgiter before TLS 1.3 optimistic retry

tlsdecryptsg() advances msg->msgiter when it maps user pages for the optimistic TLS 1.3 zero-copy path. If the decrypted record turns out not to be unpadded application data, tlsdecryptsw() retries into a kernel skb, but leaves the iterator advanced.

The subsequent copy from the skb then writes decrypted bytes again at a later point in the caller iovecs while recvmsg() reports only the post-retry length. A TLS peer can trigger this after the receiver enables TLSRXEXPECTNOPAD.

Revert the iterator by the number of bytes consumed by the optimistic mapping before retrying without zero-copy.

Add a selftest which sends a TLS 1.3 control record with TLSRXEXPECTNOPAD enabled and verifies that recvmsg() does not overwrite later iovecs beyond the returned length.