fusefrontend: Fix computation of cipherSz in Allocate FUSE call.
fusefrontend: Fix computation of cipherSz in Allocate FUSE call.

Do not use PlainSizeToCipherSize() since this adds the 18 bytes file header.

Partially fixes https://github.com/rfjakob/gocryptfs/issues/311

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