[partition] Fix up calculations of sectors in auto-partition

- Calculating first free sector had an off-by-one so that
   partitioning would start at 2049.
 - EFI boot partition grew 1 sector larger than desired.
 - While here, align everything to 1MiB boundaries as well.

FIXES #1008

src/modules/partition/core/PartitionActions.cpp

@@ -101,6 +101,24 @@ swapSuggestion( const qint64 availableSpaceB )
     return suggestedSwapSizeB;
 }
 
+constexpr qint64
+alignBytesToBlockSize( qint64 bytes, qint64 blocksize )
+{
+    Q_ASSERT( bytes >= 0 );
+    Q_ASSERT( blocksize > 0 );
+    qint64 blocks = bytes / blocksize;
+    Q_ASSERT( blocks >= 0 );
+
+    if ( blocks * blocksize != bytes )
+        ++blocks;
+    return blocks * blocksize;
+}
+
+constexpr qint64
+bytesToSectors( qint64 bytes, qint64 blocksize )
+{
+    return alignBytesToBlockSize( alignBytesToBlockSize( bytes, blocksize), MiBtoBytes(1) ) / blocksize;
+}
 
 void
 doAutopartition( PartitionCoreModule* core, Device* dev, const QString& luksPassphrase )
@@ -120,11 +138,20 @@ doAutopartition( PartitionCoreModule* core, Device* dev, const QString& luksPass
     int uefisys_part_size = isEfi ? 300 : 0;
     int empty_space_size = isEfi ? 2 : 1;
 
-    qint64 firstFreeSector = MiBtoBytes(empty_space_size) / dev->logicalSize() + 1;
+    // Since sectors count from 0, if the space is 2048 sectors in size,
+    // the first free sector has number 2048 (and there are 2048 sectors
+    // before that one, numbered 0..2047).
+    qint64 firstFreeSector = bytesToSectors( MiBtoBytes(empty_space_size), dev->logicalSize() );
 
     if ( isEfi )
     {
-        qint64 lastSector = firstFreeSector + ( MiBtoBytes(uefisys_part_size) / dev->logicalSize() );
+        qint64 efiSectorCount = bytesToSectors( MiBtoBytes(uefisys_part_size), dev->logicalSize() );
+        Q_ASSERT( efiSectorCount > 0 );
+
+        // Since sectors count from 0, and this partition is created starting
+        // at firstFreeSector, we need efiSectorCount sectors, numbered
+        // firstFreeSector..firstFreeSector+efiSectorCount-1.
+        qint64 lastSector = firstFreeSector + efiSectorCount - 1;
         core->createPartitionTable( dev, PartitionTable::gpt );
         Partition* efiPartition = KPMHelpers::createNewPartition(
             dev->partitionTable(),