Bluetooth: Remove parent socket usage from l2cap_core.c

The parent socket is not used inside the L2CAP core anymore. We only lock
it to indirect access through the new_connection() callback. The hold of
the socket lock was moved to the new_connection() callback.

Inside L2CAP core the channel lock is now used in l2cap_le_conn_ready()
and l2cap_conn_ready() to protect the execution of these two functions
during the handling of new incoming connections.

This change remove the socket lock usage from L2CAP core while keeping
the code safe against race conditions.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>