fusefrontend_reverse: Use openBackingDir in GetAttr
fusefrontend_reverse: Use openBackingDir in GetAttr

Also fixes 48bd59f38843e5ebd4e4c9f666f1aea1c9990803 - the directory FD should
also be closed in case of an error.

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