libsnapshot: Add decompression check to Inspect_Cow

This adds the -d option to Inspect_Cow, which will cause it to attempt
to decompress all data blocks, reporting any errors it encounters.
Useful for detecting corruption in Cow files.

Bug: 172026020
Test: Inspect_Cow -d [cow_file]
Change-Id: Iebf5f7f485b33b36daab4ab07005ca37e51d692f