MFC r342747 (mmacy): zfsboot: support newer ZFS versions
declare v3 objset size/layout to fix userboot and possibly other loader
issues
- fix for userboot assertion failure in zfs_dev_close in free due to out of bounds write
- fix for zfs_alloc / zfs_free mismatch assertion failure when booting GPT on BIOS
Approved by: re (gjb)