efi_gpt: properly fix block offset calculations for partition entries

In 2346363b, had corrected the offset writing to the disk, but missed
correcting the offset for reading from the entries struct.

Instead of writing a block, just write the single entry, simplifying
the offset logic considerably.

src/add-ons/kernel/partitioning_systems/gpt/Header.cpp

@@ -193,14 +193,11 @@ Header::InitCheck() const
 status_t
 Header::WriteEntry(int fd, uint32 entryIndex)
 {
-	// Determine block to write
-	off_t blockOffset =
-		+ entryIndex * fHeader.EntrySize() / fBlockSize;
-	uint32 entryOffset = entryIndex * fHeader.EntrySize() % fBlockSize;
+	off_t entryOffset = entryIndex * fHeader.EntrySize();
 
 	status_t status = _Write(fd,
-		(fHeader.EntriesBlock() + blockOffset) * fBlockSize + entryOffset,
-		fEntries + entryOffset, fBlockSize);
+		fHeader.EntriesBlock() * fBlockSize + entryOffset,
+		fEntries + entryOffset, fHeader.EntrySize());
 	if (status != B_OK)
 		return status;
 
@@ -209,8 +206,8 @@ Header::WriteEntry(int fd, uint32 entryIndex)
 
 	// Write backup
 	status_t backupStatus = _Write(fd,
-		(fBackupHeader.EntriesBlock() + blockOffset) * fBlockSize + entryOffset,
-		fEntries + entryOffset, fBlockSize);
+		fBackupHeader.EntriesBlock() * fBlockSize + entryOffset,
+		fEntries + entryOffset, fHeader.EntrySize());
 
 	return status == B_OK ? backupStatus : status;
 }