OpenZFS 6286 - ZFS internal error when set large block on bootfs

6286 ZFS internal error when set large block on bootfs
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6286
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/6de9bb5
Closes #4585