[SCSI] scsi_scan: Cope with kthread_run failing

If kthread_run failed, we would fail to scan the host, and leak the
allocated async_scan_data.  Since using a separate thread is just an
optimisation, do the scan synchronously if we fail to spawn a thread.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>