Bluetooth: Mesh: Allow to suspend mesh from bt_mesh_send_cb callbacks

This commit allows to suspend the mesh stack from `bt_mesh_send_cb`
callbacks by removing the deadlock caused by `k_work_flush` in the
extended advertiser.

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.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>