Bluetooth: L2CAP: Fix use-after-free in l2cap_disconnect_{req,rsp}

Similar to commit d0be8347 ("Bluetooth: L2CAP: Fix use-after-free
caused by l2cap_chan_put"), just use l2cap_chan_hold_unless_zero to
prevent referencing a channel that is about to be destroyed.

Cc: stable@kernel.org
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Min Li <lm0963hack@gmail.com>