ZFS: fix the redundancy tests

• Fix force_sync_path, which ensures that a file is fully flushed to disk. Apparently "zpool history"'s performance has improved, but exporting and importing the pool still works.
• Fix file_dva by using undocumented zdb syntax to clarify that we're interested in the pool's root file system, not the pool itself. This should also fix the zpool_clear_001_pos test.
• Remove a redundant cleanup step

MFC after: 2 weeks
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D21901