Build regression in c89 cleanups

Fixed build regression in non-debug builds from recent cleanups of
c89 workarounds.

Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #6832