Fix dsl_prop_get_all_dsl() memory leak

On error dsl_prop_get_all_ds() does not free the nvlist it allocates.
This behavior may have been intentional when originally written
but is atypical and often confusing. Since no callers rely on this
behavior the function has been updated to always free the nvlist
on error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: BearBabyLiu <liu.huang@zte.com.cn>
Closes #5320