fusefs: Don't treat fsync the same as fdatasync

For an unknown reason, fusefs was _always_ sending the fdatasync operation
instead of fsync. Now it correctly sends one or the other.

Also, remove the Fsync.fsync_metadata_only test, along with the recently
removed Fsync.nop. They should never have been added. The kernel shouldn't
keep track of which files have dirty data; that's the daemon's job.

PR: 236473
Sponsored by: The FreeBSD Foundation