Silence unused-but-set-variable warning

This was breaking the kmod port build on FreeBSD with Clang 13.

Use the same trick as we do for ASSERT() to make DNODE_VERIFY() use
its parameter at compile time without actually using it at run time
in non-debug builds.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #13507