Fix module build with -Werror

This is a direct commit to zfs-2.1-release to fix release builds that
error out on an unused variable. The issue is avoided on master by a
huge series of commits that change how the ASSERT macros work, but that
is not feasible to backport.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #13194
Closes #13196