char: xillybus: Fix trivial bug with mutex

@unit_mutex protects @unit from being freed, so obviously it should be
released after @unit is used, and not before.

This is a follow-up to commit 282a4b71816b ("char: xillybus: Prevent
use-after-free due to race condition") which ensures, among others, the
protection of @private_data after @unit_mutex has been released.

Reported-by: Hyunwoo Kim <[e-mail]>
Signed-off-by: Eli Billauer <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/xillybus/xillybus_class.c

···
 			break;
 		}
 
-	mutex_unlock(&unit_mutex);
-
-	if (!unit)
+	if (!unit) {
+		mutex_unlock(&unit_mutex);
 		return -ENODEV;
+	}
 
 	*private_data = unit->private_data;
 	*index = minor - unit->lowest_minor;
 
+	mutex_unlock(&unit_mutex);
 	return 0;
 }
 EXPORT_SYMBOL(xillybus_find_inode);