tty: n_gsm: require CAP_NET_ADMIN to attach N_GSM0710 ldisc

tty: n_gsm: require CAP_NET_ADMIN to attach N_GSM0710 ldisc

commit 67c37756898a5a6b2941a13ae7260c89b54e0d88 upstream.

Any unprivileged user can attach N_GSM0710 ldisc, but it requires
CAP_NET_ADMIN to create a GSM network anyway.

Require initial namespace CAP_NET_ADMIN to do that.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Link: https://lore.kernel.org/r/20230731185942.279611-1-cascardo@canonical.com
Cc: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>