During driver unload, the driver may crash due to NULL pointers.  The
NULL pointers were due to the driver not protecting itself sufficiently
during some of the teardown paths.  Additionally, the driver was not
waiting for and cleanup up nvme io resources. As such, the driver wasn't
making the callbacks to the transport, stalling the transports
association teardown.

This patch waits for io clean up before tearding down and adds checks
for possible NULL pointers.

Cc: <stable@vger.kernel.org> # 4.12+
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>