scsi: revert change to scsi_req_cancel_async and add assertions

Fam Zheng noticed that the change in commit 36896bf ("scsi: always call
notifier on async cancellation", 2015-12-16) could cause a leak of
the request; scsi_req_cancel_async now calls scsi_req_ref
multiple times for multiple cancellations, but there is only
one call to scsi_req_cancel_complete.

So revert the patch and instead assert that the problematic case (a call
to scsi_req_cancel_async after the aiocb has been completed) cannot
happen.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
