ntb: stop tasklet from spinning forever during shutdown.

We can leave tasklet spinning forever if we disable the tasklet during
qp shutdown and the tasklets are still being kicked off. This hopefully
should avoid that race condition.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reported-by: Alex Depoutovitch <alex@pernixdata.com>
Tested-by: Alex Depoutovitch <alex@pernixdata.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>