fusefs: fix fsync for files with multiple open handles

We were reusing a structure for multiple operations, but failing to
reinitialize one member. The result is that a server that cares about FUSE
file handle IDs would see one correct FUSE_FSYNC operation, and one with the
FHID unset.

PR: 244431
Reported by: Agata <chogata@gmail.com>
MFC after: 2 weeks