Bluetooth: Mesh: Allow to suspend mesh from bt_mesh_send_cb callbacks

[PavelVPV]

This PR allows to suspend the mesh stack from bt_mesh_send_cb callback by removing the deadlock caused by k_work_flush.

In case of the extended advertiser there are 2 cases:

• when the bt_mesh_adv_disable is called from any of bt_mesh_send_cb callbacks which are called from the advertiser work item, or
• when it is called from any other context.

When it is called from bt_mesh_send_cb callbacks, since these callbacks are called from the delayable work which is running on the system workqueue, the advertiser can check the current context and its work state. If the function is called from the advertiser work, it can disable the advertising set straight away because all ble host APIs have already been called in adv_start function. Before sending anything else, the advertiser checks the instance value in adv_start function, which is also reset to NULL in bt_mesh_adv_disable call, and aborts all next advertisements. The ADV_FLAG_SUSPENDING tells the advertiser work to abort processing while bt_mesh_adv_disable function didn't finish stopping advertising set. This can happen if the work has been already scheduled and the schedler ran it while sleeping inside the bt_le_ext_adv_stop or bt_le_ext_adv_disable functions.

When bt_mesh_adv_disable is called from any other context or from the system workqueue but not from the advertiser work, then k_work_flush can be called safely as it won't cause any deadlocks.

The adv_sent function is inside the bt_mesh_adv_disable function to schedule the advertiser work (send_pending_adv) and abort all pending advertisements that have been already added to the pool.

In case of the legacy advertiser, if the bt_mesh_adv_disable is called form the advertiser thread (this happens when it is called from bt_mesh_send_cb.start or bt_mesh_send_cb.end callbacks), then k_thread_join returns -EDEADLK. But the enabled flag is set to false and the thread will abort the current advertisement and the pending advertisements.