4188 assertion failed in dmu_tx_hold_free(): dn_datablkshift != 0

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>

Refences:

https://www.illumos.org/issues/4188
illumos/illumos-gate@bb411a08b05466bfe0c7095b6373bbc1587e259a

Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2091