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>

diff --cc drivers/target/target_core_device.c
Simple merge

diff --cc drivers/target/target_core_internal.h
Simple merge

diff --cc drivers/target/target_core_tpg.c
Simple merge

diff --cc drivers/target/target_core_transport.c
Simple merge

diff --cc include/target/target_core_base.h
Simple merge