Bluetooth: mgmt: Introduce mgmt_alloc_skb and mgmt_send_event_skb

This introduces mgmt_alloc_skb and mgmt_send_event_skb which are
convenient when building MGMT events that have variable length as the
likes of skb_put_data can be used to insert portion directly on the skb
instead of having to first build an intermediate buffer just to be
copied over the skb.

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