config: fix page_mapping test

It always failed from "unused variable" warnings-errors. The resulting
#define page_mapping(...) happend to work because it always overrode
the kernel's function prototype, but that's brittle.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16479