target: Simplify LUN shutdown code

Instead of starting a thread from transport_clear_lun_ref() that
waits for LUN shutdown, wait in that function for LUN shutdown
to finish. Additionally, change the return type of
transport_clear_lun_ref() from int to void.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>