Prevent ZFS TRIM breaking VTOC8 partitions

Update the ZFS TRIM code to ensure it respects VTOC8 partition headers as
documented by the ZFS On-Disk Specification section 1.3

Before this a zpool create on a VTOC8 partitioned device would overwrite the
partition metadata.

Reported by: marius
Reviewed by: marius agv
MFC after: 1 week
Sponsored by: Multiplay