[fsresizer] Simplify and make safer

- Make RelativeSize public so we can use it in non-member functions
 - Make a template out of matching the string suffixes; this is
   safer because the length of the suffix can be computed at compile-time
   (+1 for the trailing NUL) rather than writing it out in boilerplate.

src/modules/fsresizer/ResizeFSJob.cpp

@@ -29,32 +29,36 @@
 #include "utils/Logger.h"
 
 ResizeFSJob::RelativeSize::RelativeSize()
-    : m_unit( None )
-    , m_value( 0 )
+    : m_value( 0 )
+    , m_unit( None )
 {
 }
 
 
-ResizeFSJob::RelativeSize::RelativeSize( const QString& s)
-    : m_unit( None )
-    , m_value( 0 )
+template<int N>
+void matchUnitSuffix( 
+    const QString& s, 
+    const char (&suffix)[N],
+    ResizeFSJob::RelativeSize::Unit matchedUnit,
+    int& value,
+    ResizeFSJob::RelativeSize::Unit& unit
+     )
 {
-    QString valuePart;
-
-    if ( s.endsWith( '%' ) )
+    if ( s.endsWith( suffix ) )
     {
-        valuePart = s.left( s.length() - 1 );
-        m_unit = Percent;
+        value = s.left( s.length() - N + 1 ).toInt();
+        unit = matchedUnit;
     }
-    if ( s.endsWith( "MiB" ) )
-    {
-        valuePart = s.left( s.length() - 3 );
-        m_unit = Absolute;
-    }
-
-    if ( ( m_unit != None ) && !valuePart.isEmpty() )
-        m_value = valuePart.toInt();
-
+}
+        
+    
+ResizeFSJob::RelativeSize::RelativeSize( const QString& s)
+    : m_value( 0 )
+    , m_unit( None )
+{
+    matchUnitSuffix( s, "%", Percent, m_value, m_unit );
+    matchUnitSuffix( s, "MiB", Absolute, m_value, m_unit );
+    
     if ( !m_value )
         m_unit = None;
 }

src/modules/fsresizer/ResizeFSJob.h

@@ -32,6 +32,7 @@ class PLUGINDLLEXPORT ResizeFSJob : public Calamares::CppJob
 {
     Q_OBJECT
 
+public:
     class RelativeSize
     {
     public:
@@ -45,15 +46,14 @@ class PLUGINDLLEXPORT ResizeFSJob : public Calamares::CppJob
             Absolute
         };
 
-        quint64 value() const { return m_value; }
+        int value() const { return m_value; }
         Unit unit() const { return m_unit; }
 
     private:
-        quint64 m_value;
+        int m_value;
         Unit m_unit;
     } ;
 
-public:
     explicit ResizeFSJob( QObject* parent = nullptr );
     virtual ~ResizeFSJob() override;