Inverted layer checksum and tarsum.

The checksum of the payload has to be computed on the Gzip'ed content.

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Payload checksum now match the checksum simple

Backported for backward compatibility.

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Merge pull request #4945 from samalba/fix-checksum-layer

Fix checksum layer