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 282a4b71 ("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 <imv4bel@gmail.com>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20221117071825.3942-1-eli.billauer@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/xillybus/xillybus_class.c

@@ -227,14 +227,15 @@ int xillybus_find_inode(struct inode *inode,
 			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);