zfs: fix the slog_012_neg test

This test attempts to corrupt a file-backed vdev by deleting it and then
recreating it with truncate. But that doesn't work, because the pool
already has the vdev open, and it happily hangs on to the open-but-deleted
file. Fix by truncating the file without deleting it.

MFC after: 2 weeks
Sponsored by: Axcient