tcmu: split unmap_thread_fn

Separate unmap_thread_fn to make it easier to read.

Note: this patch does not fix the bug where we might
miss a wake up call. The next patch will fix that.
This patch only separates the code into functions.

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