Illumos 5141 - zfs minimum indirect block size is 4K

Illumos 5141 - zfs minimum indirect block size is 4K

5141 zfs minimum indirect block size is 4K
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Richard Elling <richard.elling@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/5141
  https://github.com/illumos/illumos-gate/commit/e94f268

Porting notes:
- GRUB  --  GRand Unified Bootloader change wasn't merged (not applicable)

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>