selftests: net/psock_fanout: socket joins fanout when link is down

Modify test_control_group to have toggle parameter.
When toggle is non-zero, loopback device will be set down for the
initialization of fd[1] which is still expected to successfully join
the fanout.

Signed-off-by: Gur Stavi <gur.stavi@huawei.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/6f4a506ed5f08f8fc00a966dec8febd1030c6e98.1728802323.git.gur.stavi@huawei.com


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