KVM: selftests: Fix nsec to sec conversion in demand_paging_test

demand_paging_test uses 1E8 as the denominator to convert nanoseconds to
seconds, which is wrong. Use NSEC_PER_SEC instead to fix the issue and
make the conversion obvious.

Reported-by: James Houghton <jthoughton@google.com>
Signed-off-by: Anish Moorthy <amoorthy@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20230223001805.2971237-1-amoorthy@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/demand_paging_test.c

@@ -194,7 +194,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 		ts_diff.tv_sec, ts_diff.tv_nsec);
 	pr_info("Overall demand paging rate: %f pgs/sec\n",
 		memstress_args.vcpu_args[0].pages * nr_vcpus /
-		((double)ts_diff.tv_sec + (double)ts_diff.tv_nsec / 100000000.0));
+		((double)ts_diff.tv_sec + (double)ts_diff.tv_nsec / NSEC_PER_SEC));
 
 	memstress_destroy_vm(vm);