Provisioner should always uses the latest IV Index for provisioning.
For example, if the current IV Index is 0x00000001, but prov->iv_index
is still initialized with 0x00000000, and if Provisioner uses prov->
iv_index for provisioning, this will cause the Provisioner failing to
control the node.
So here bt_mesh.iv_index is used instead of prov->iv_index.