Bluetooth: Provide msg_name callback for L2CAP connectionless channels

The L2CAP connectionless channels use SOCK_DGRAM and recvmsg() and need
to receive the remote BD_ADDR and PSM information via msg_name from
the recvmsg() system call.

So in case the L2CAP socket is for connectionless channels, provide
a msg_name callback that can update the data. Also store the remote
BD_ADDR and PSM in the skb so it can be extracted later on.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>