target: Remove extra percpu_ref_init

lun->lun_ref is also initialized in core_tpg_post_addlun, so it doesn't
need to be done in core_tpg_setup_virtual_lun0.

(nab: Drop left-over percpu_ref_cancel_init in failure path)

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>