vsock/test: Add test for null ptr deref when transport changes

vsock/test: Add test for null ptr deref when transport changes

Add a new test to ensure that when the transport changes a null pointer
dereference does not occur. The bug was reported upstream [1] and fixed
with commit 2cb7c756f605 ("vsock/virtio: discard packets if the
transport changes").

KASAN: null-ptr-deref in range [0x0000000000000060-0x0000000000000067]
CPU: 2 UID: 0 PID: 463 Comm: kworker/2:3 Not tainted
Workqueue: vsock-loopback vsock_loopback_work
RIP: 0010:vsock_stream_has_data+0x44/0x70
Call Trace:
 virtio_transport_do_close+0x68/0x1a0
 virtio_transport_recv_pkt+0x1045/0x2ae4
 vsock_loopback_work+0x27d/0x3f0
 process_one_work+0x846/0x1420
 worker_thread+0x5b3/0xf80
 kthread+0x35a/0x700
 ret_from_fork+0x2d/0x70
 ret_from_fork_asm+0x1a/0x30

Note that this test may not fail in a kernel without the fix, but it may
hang on the client side if it triggers a kernel oops.

This works by creating a socket, trying to connect to a server, and then
executing a second connect operation on the same socket but to a
different CID (0). This triggers a transport change. If the connect
operation is interrupted by a signal, this could cause a null-ptr-deref.

Since this bug is non-deterministic, we need to try several times. It
is reasonable to assume that the bug will show up within the timeout
period.

If there is a G2H transport loaded in the system, the bug is not
triggered and this test will always pass. This is because
`vsock_assign_transport`, when using CID 0, like in this case, sets
vsk->transport to `transport_g2h` that is not NULL if a G2H transport is
available.

[1]https://lore.kernel.org/netdev/Z2LvdTTQR7dBmPb5@v4bel-B760M-AORUS-ELITE-AX/

Suggested-by: Hyunwoo Kim <v4bel@theori.io>
Suggested-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250630-test_vsock-v5-2-2492e141e80b@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>