config/kmap_atomic: initialise test data

6.18 changes kmap_atomic() to take a const pointer. This is no problem
for the places we use it, but Clang fails the test due to a warning
about being unable to guarantee that uninitialised data will definitely
not change. Easily solved by forcibly initialising it.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17954