[SCSI] scsi_scan.c: Release mutex in error handling code

The mutex is released on a successful return, so it would seem that it
should be released on an error return as well.

The semantic patch that makes this change is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@@
expression l;
@@

mutex_lock(l);
... when != mutex_unlock(l)
    when any
    when strict
(
if (...) { ... when != mutex_unlock(l)
+   mutex_unlock(l);
    return ...;
}
|
mutex_unlock(l);
)
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/scsi_scan.c

@@ -1760,6 +1760,7 @@ static void scsi_finish_async_scan(struct async_scan_data *data)
 		printk("%s called twice for host %d", __FUNCTION__,
 				shost->host_no);
 		dump_stack();
+		mutex_unlock(&shost->scan_mutex);
 		return;
 	}