ztest: display non-index properties properly at verbose level 6

ztest: display non-index properties properly at verbose level 6

At verbosity levels of 6 or greater, ztest_dsl_prop_set_uint64() attempts
to display the value of all properties as indexed values regardless of
whether the property is an indexed value or simply an un-indexed integer.
This patch causes the numeric value of the property to be displayed if
zfs_prop_index_to_string() fails.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3649