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>
Author: Matthew Ahrens <mahrens@delphix.com>

illumos/illumos-gate@6de9bb5603e65b16816b7ab29e39bac820e2da2b