Bluetooth: Add support for per socket msg_name callback

This allows to add a per socket msg_name callback that can be used
for updating the msg_name information for recvmsg() system calls.

This feature is used by another patch to support address information
on L2CAP connectionless channels.

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