libfuse: don't use double-pointer ctx app id
libfuse: don't use double-pointer ctx app id

At some point NewFS was changed to return a pointer.

Issue: KBFS-661

keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client