MFV r254071:

Fix a regression introduced by fix for Illumos bug #3834. Quote from
Matthew Ahrens on the Illumos issue:

ztest fails this assertion because ztest_dmu_read_write() does

dmu_tx_hold_free(tx, bigobj, bigoff, bigsize);

and then

dmu_object_set_checksum(os, bigobj,
    (enum zio_checksum)ztest_random_dsl_prop(ZFS_PROP_CHECKSUM), tx);

If the region to free is past the end of the file, the DMU assumes that there
will be nothing to do for this object. However, ztest does set_checksum(),
which must modify the dnode. The fix is for ztest to also call

dmu_tx_hold_bonus(tx, bigobj);

so we can account for the dirty data associated with setting the checksum

Illumos ZFS issues:

3955 ztest failure: assertion refcount_count(&tx->tx_space_written)
       + delta <= tx->tx_space_towrite