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

include/sys/dnode.h

@@ -616,7 +616,7 @@ extern dnode_stats_t dnode_stats;
 #else
 
 #define	dprintf_dnode(db, fmt, ...)
-#define	DNODE_VERIFY(dn)
+#define	DNODE_VERIFY(dn)		((void) sizeof ((uintptr_t)(dn)))
 #define	FREE_VERIFY(db, start, end, tx)
 
 #endif