tcmu: fix module removal due to stuck thread

We need to do a kthread_should_stop to check when kthread_stop has been
called.

This was a regression added in

b6df4b79
tcmu: Add global data block pool support

so not sure if you wanted to merge it in with that patch or what.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>