NTB: Fix incorrect clean up routine in ntb_perf

The clean up routine when we failed to allocate kthread is not cleaning
up all the threads, only the same one over and over again.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Acked-by: Allen Hubbe <Allen.Hubbe@emc.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>