AU: Add a utility routine to get the filesystem size from a device.

utils.h

Index: utils.h
diff --git a/utils.h b/utils.h
index bfe73784d9d940e971ed4330d84eba3ff1ece04c..9beae04b9bb025ed1782c57db4efc7562e49c4e0 100644
--- a/utils.h
+++ b/utils.h
@@ -115,6 +115,17 @@ bool MountFilesystem(const std::string& device, const std::string& mountpoint,
                      unsigned long flags);
 bool UnmountFilesystem(const std::string& mountpoint);
 
+// Returns the block count and the block byte size of the ext3 file system on
+// |device| (which may be a real device or a path to a filesystem image) or on
+// an opened file descriptor |fd|. The actual file-system size is |block_count|
+// * |block_size| bytes. Returns true on success, false otherwise.
+bool GetFilesystemSize(const std::string& device,
+                       int* out_block_count,
+                       int* out_block_size);
+bool GetFilesystemSizeFromFD(int fd,
+                             int* out_block_count,
+                             int* out_block_size);
+
 enum BootLoader {
   BootLoader_SYSLINUX = 0,
   BootLoader_CHROME_FIRMWARE = 1