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/[e-mail]/

Signed-off-by: Michal Luczaj <[e-mail]>
Reviewed-by: Stefano Garzarella <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/vsock/vsock_test.c

···
 			exit(EXIT_FAILURE);
 		}
 
+		/* Although setting SO_LINGER does not affect the original test
+		 * for null-ptr-deref, it may trigger a lockdep warning.
+		 */
+		enable_so_linger(s, 1);
+
 		ret = connect(s, (struct sockaddr *)&sa, sizeof(sa));
 		/* The connect can fail due to signals coming from the thread,
 		 * or because the receiver connection queue is full.
···
 		.run_server = test_stream_nolinger_server,
 	},
 	{
-		.name = "SOCK_STREAM transport change null-ptr-deref",
+		.name = "SOCK_STREAM transport change null-ptr-deref, lockdep warn",
 		.run_client = test_stream_transport_change_client,
 		.run_server = test_stream_transport_change_server,
 	},