Add new API dp_vdev_get_ref_by_id() which will return vdev
pointer by holding a reference. Caller of this API has
to ensure that this reference is released by calling
dp_vdev_unref_delete() API
New lock soc->vdev_map_lock is introduced to protect
vdev id to object array
Change-Id: I883e328932e35ef31254125492dbae20cebe0e00