bootloader: Make Partition::Size return the partition's size.

Partition::ReadAt() and WriteAt() already do bounds-checking
based on the size, but Size() was returning the underlying FD
size, which might be that of the entire disk, not just this
partition. So, move initialization around a bit, and return
the actual size.

Should fix the bootloader menu displaying the wrong sizes
for boot partitions.

src/system/boot/loader/partitions.cpp

@@ -216,11 +216,7 @@ Partition::WriteAt(void *cookie, off_t position, const void *buffer,
 off_t
 Partition::Size() const
 {
-	struct stat stat;
-	if (fstat(fFD, &stat) == B_OK)
-		return stat.st_size;
-
-	return Node::Size();
+	return size;
 }
 
 
@@ -452,7 +448,10 @@ add_partitions_for(int fd, bool mountFileSystems, bool isBootDevice)
 
 	// set some magic/default values
 	partition->block_size = 512;
-	partition->size = partition->Size();
+
+	struct stat stat;
+	if (fstat(fd, &stat) == B_OK)
+		partition->size = stat.st_size;
 
 	// add this partition to the list of partitions
 	// temporarily for Lookup() to work