[partition] Use TemporaryMount support class

- While here, fix up compile warning
 - add -R to umount for good measure

src/libcalamares/partition/Mount.cpp

@@ -96,7 +96,7 @@ TemporaryMount::~TemporaryMount()
 {
     if ( m_valid )
     {
-        unmount( m_devicePath );
+        unmount( m_devicePath, { "-R" } );
     }
 }
 

src/libcalamares/partition/Mount.h

@@ -54,7 +54,7 @@ DLLEXPORT int mount( const QString& devicePath,
  */
 DLLEXPORT int unmount( const QString& path, const QStringList& options = QStringList() );
 
-DLLEXPORT class TemporaryMount
+class DLLEXPORT TemporaryMount
 {
 public:
     TemporaryMount( const QString& devicePath,

src/modules/partition/core/PartUtils.cpp

@@ -28,6 +28,7 @@
 
 #include "GlobalStorage.h"
 #include "JobQueue.h"
+#include "partition/Mount.h"
 #include "partition/PartitionIterator.h"
 #include "partition/PartitionQuery.h"
 #include "utils/CalamaresUtilsSystem.h"
@@ -248,8 +249,8 @@ lookForFstabEntries( const QString& partitionPath )
     QTemporaryDir mountsDir;
     mountsDir.setAutoRemove( false );
 
-    int exit = QProcess::execute( "mount", { "-o", mountOptions.join(','), partitionPath, mountsDir.path() } );
-    if ( !exit ) // if all is well
+    CalamaresUtils::Partition::TemporaryMount mount( partitionPath, mountsDir.path(), QString(), mountOptions.join(',') );
+    if ( mount.isValid() )
     {
         QFile fstabFile( mountsDir.path() + "/etc/fstab" );
 
@@ -269,9 +270,6 @@ lookForFstabEntries( const QString& partitionPath )
         }
         else
             cWarning() << "Could not read fstab from mounted fs";
-
-        if ( QProcess::execute( "umount", { "-R", mountsDir.path() } ) )
-            cWarning() << "Could not unmount" << mountsDir.path();
     }
     else
         cWarning() << "Could not mount existing fs";