[gparted] Replace open coding FS unknown usage check in prepare_new_partition()

[Curtis Gedak <gedakc src gnome org>]

commit 18bac7c92750e8aef23b40b086eda9e41c9a9816
Author: Mike Fleetwood <mike fleetwood googlemail com>
Date:   Wed Sep 5 07:36:20 2018 +0100

    Replace open coding FS unknown usage check in prepare_new_partition()
    
    Back when unallocated space handling was being added, this case was not
    converted from open coding to using the provided method to check for
    unknown file system usage.  Specifically this commit missed using
    Partition::sector_usage_known() in
    Dialog_Base_Partition::prepare_new_partition():
    
        7ebedc4bb3b81e85fb4c628a2a05308ada147d68
        Don't show intrinsic unallocated space (#499202)
    
    Fix it now.

 src/Dialog_Base_Partition.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/Dialog_Base_Partition.cc b/src/Dialog_Base_Partition.cc
index aeaae9a0..70f0687b 100644
--- a/src/Dialog_Base_Partition.cc
+++ b/src/Dialog_Base_Partition.cc
@@ -210,7 +210,7 @@ void Dialog_Base_Partition::prepare_new_partition()
        }
 
        //update partition usage
-       if ( new_partition->sectors_used != -1 && new_partition->sectors_unused != -1 )
+       if ( new_partition->sector_usage_known() )
        {
                Sector new_size = new_partition->get_sector_length();
                if ( old_size == new_size )