fusefrontend: Properly convert plaintext <-> ciphertext offsets in SeekData().
fusefrontend: Properly convert plaintext <-> ciphertext offsets in SeekData().

Fixes https://github.com/rfjakob/gocryptfs/issues/304

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