Bluetooth: L2CAP: Fix not validating setsockopt user input

Check user input length before copying data.

Fixes: 33575df7 ("Bluetooth: move l2cap_sock_setsockopt() to l2cap_sock.c")
Fixes: 3ee7b7cd ("Bluetooth: Add BT_MODE socket option")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>