Bluetooth: L2CAP: Fix not initializing sk_peer_pid

In order to group sockets being connected using L2CAP_MODE_EXT_FLOWCTL
the pid is used but sk_peer_pid was not being initialized as it is
currently only done for af_unix.

Fixes: b48596d1 ("Bluetooth: L2CAP: Add get_peer_pid callback")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>