fusefrontend: -allow_other: set file mode *after* chown in Create()

Reported by @slackner at https://github.com/rfjakob/gocryptfs/issues/327 :

Possible race-conditions between file creation and Fchownat

* Assume a system contains a gocryptfs mount as root user
  with -allow_other
* As a regular user create a new file with mode containing
  the SUID flag and write access for other users
* Before gocryptfs executes the Fchownat call, try to open
  the file again, write some exploit code to it, and try to run it.

For a short time, the file is owned by root and has the SUID flag, so
this is pretty dangerous.

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