blkg_tryget config test: initialize struct

Missing struct initialization in a config test results in the
interface being incorrectly detected.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Adam Moss <c@yotes.com>
Signed-off-by: Mathieu Velten <matmaul@gmail.com>
Closes #10713
Closes #11049