Bluetooth: Reorganize mutex lock in l2cap_sock_shutdown()

Bluetooth: Reorganize mutex lock in l2cap_sock_shutdown()

This commit reorganizes the mutex lock and is now
only protecting l2cap_chan_close(). This is now consistent
with other places where l2cap_chan_close() is called.

If a conn connection exists, call
mutex_lock(&conn->chan_lock) before calling l2cap_chan_close()
to ensure other L2CAP protocol operations do not interfere.

Note that the conn structure has to be protected from being
freed as it is possible for the connection to be disconnected
whilst the locks are not held. This solution allows the mutex
lock to be used even when the connection has just been
disconnected.

This commit also reduces the scope of chan locking.

The only place where chan locking is needed is the call to
l2cap_chan_close(chan, 0) which if necessary closes the channel.
Therefore, move the l2cap_chan_lock(chan) and
l2cap_chan_lock(chan) locking calls to around
l2cap_chan_close(chan, 0).

This allows __l2cap_wait_ack(sk, chan) to be called with no
chan locks being held so L2CAP messaging over the ACL link
can be done unimpaired.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Signed-off-by: Harish Jenny K N <harish_kandiga@mentor.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>