vsock/test: Extend transport change null-ptr-deref test

vsock/test: Extend transport change null-ptr-deref test

syzkaller reported a lockdep lock order inversion warning[1] due to
commit 687aa0c5581b ("vsock: Fix transport_* TOCTOU"). This was fixed in
commit f7c877e75352 ("vsock: fix lock inversion in
vsock_assign_transport()").

Redo syzkaller's repro by piggybacking on a somewhat related test
implemented in commit 3a764d93385c ("vsock/test: Add test for null ptr
deref when transport changes").

[1]: https://lore.kernel.org/netdev/68f6cdb0.a70a0220.205af.0039.GAE@google.com/

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20251123-vsock_test-linger-lockdep-warn-v1-1-4b1edf9d8cdc@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>