Illumos #3955

3955 ztest failure: assertion refcount_count(&tx->tx_space_written) +

delta <= tx->tx_space_towrite

Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:

https://www.illumos.org/issues/3955
illumos/illumos-gate@be9000cc677e0a8d04e5be45c61d7370fc8c7b54

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1775