ext2fs: Fix optimization bug for doubly-indirect block pointers

Doubly-indirect block numbers are compared against the first-level
indirect block when checking for a cached copy.  This is causing the
doubly-indirect block to be re-read each time it is accessed.
Repairing this reduces load time for a 70M file from 72 seconds
to 38 seconds.

Signed-off-by: Aaron Pace <Aaron.Pace@alcatel-lucent.com>