[PATCH net v4 0/4] vsock/virtio: fix TX credit handling

This series fixes TX credit handling in virtio-vsock:

Patch 1: Fix potential underflow in get_credit() using s64 arithmetic
Patch 2: Cap TX credit to local buffer size (security hardening)
Patch 3: Fix vsock_test seqpacket bounds test
Patch 4: Add stream TX credit bounds regression test

The core issue is that a malicious guest can advertise a huge buffer
size via SO_VM_SOCKETS_BUFFER_SIZE, causing the host to allocate
excessive sk_buff memory when sending data to that guest.

On an unpatched Ubuntu 22.04 host (~64 GiB RAM), running a PoC with
32 guest vsock connections advertising 2 GiB each and reading slowly
drove Slab/SUnreclaim from ~0.5 GiB to ~57 GiB; the system only
recovered after killing the QEMU process.

With this series applied, the same PoC shows only ~35 MiB increase in
Slab/SUnreclaim, no host OOM, and the guest remains responsive.
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2.34.1