vsock/test: Add test for SO_LINGER null ptr deref

Explicitly close() a TCP_ESTABLISHED (connectible) socket with SO_LINGER
enabled.

As for now, test does not verify if close() actually lingers.
On an unpatched machine, may trigger a null pointer dereference.

Tested-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20250210-vsock-linger-nullderef-v3-2-ef6244d02b54@rbox.co


Signed-off-by: Jakub Kicinski <kuba@kernel.org>