Bluetooth: L2CAP: refuse __l2cap_chan_add if chan already has conn

l2cap_chan may be linked to l2cap_conn at most once. This is assumed in
several places, eg l2cap_chan_del cleanup.

There is a TOCTOU race where the invariant is violated:

    [Task 1]                          [Task 2]
    l2cap_chan_connect                l2cap_sock_bind
      l2cap_chan_lock                   lock_sock
      l2cap_state_change                if (sk->sk_state != BT_OPEN)
        chan->state = BT_CONNECT
        l2cap_sock_state_change_cb      chan->state = BT_BOUND
                                        sk->sk_state = BT_BOUND
          lock_sock <------------------ release_sock
          sk->sk_state = BT_CONNECT

l2cap_sock_connect() does not check sk->sk_state, so since chan->state
is now BT_BOUND, subsequent connect() ends up with second
__l2cap_chan_add.

Explicitly document and check the invariant in __l2cap_chan_add with
WARN_ON_ONCE. The only callsite where it could be hit is
l2cap_chan_connect, so add pre-check there to avoid relying on
chan->state. chan->state read/write is not properly guarded currently so
there can be other TOCTOUC problems.

Add l2cap_lock_chan in l2cap_sock_bind() to guard chan->state write.

Fixes: b66774b48dd9 ("Bluetooth: L2CAP: Fix UAF in channel timeout by holding conn ref")
Assisted-by: deepseek-4-flash # finding the race condition
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>