contentenc: deduplicate AD packing into new concatAD() func
contentenc: deduplicate AD packing into new concatAD() func

The encrypt and decrypt path both had a copy that were equivalent
but ordered differently, which was confusing.

Consolidate it in a new dedicated function.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs