MFC r307317: MFV r307313:
5120 zfs should allow large block/gzip/raidz boot pool (loader project)

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Andrew Stormont <andyjstormont@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Toomas Soome <tsoome@me.com>

openzfs/openzfs@c8811bd3e2427dddbac6c05a59cfe117d8fea370

FreeBSD still does not support booting from gzip-compressed datasets,
so keep one chunk of this commit out.