xen/scsiback: Make TMF processing slightly faster

Target drivers must guarantee that struct se_cmd and struct se_tmr_req
exist as long as target_tmr_work() is in progress. Since the last
access by the LIO core is a call to .check_stop_free() and since the
Xen scsiback .check_stop_free() drops a reference to the TMF, it is
already guaranteed that the struct se_cmd that corresponds to the TMF
exists as long as target_tmr_work() is in progress. Hence change the
second argument of transport_generic_free_cmd() from 1 into 0.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: David Disseldorp <ddiss@suse.de>
Cc: xen-devel@lists.xenproject.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>