Bluetooth: Add bt_skb_sendmsg helper

bt_skb_sendmsg helps takes care of allocation the skb and copying the
the contents of msg over to the skb while checking for possible errors
so it should be safe to call it without holding lock_sock.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>