Bluetooth: hci_ldisc: require CAP_NET_ADMIN to attach N_HCI ldisc

Bluetooth: hci_ldisc: require CAP_NET_ADMIN to attach N_HCI ldisc

BugLink: https://bugs.launchpad.net/bugs/1949516
Any unprivileged user can attach N_HCI ldisc and send packets coming from a
virtual controller by using PTYs.

Require initial namespace CAP_NET_ADMIN to do that.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
(cherry picked from commit c05731d0c6bd9a625e27ea5c5157ebf1303229e0)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>