KVM: selftests: Fix inverted "warning" in access tracking perf test

Warn if the number of idle pages is greater than or equal to 10% of the
total number of pages, not if the percentage of idle pages is less than
10%.  The original code asserted that less than 10% of pages were still
idle, but the check got inverted when the assert was converted to a
warning.

Opportunistically clean up the warning; selftests are 64-bit only, there
is no need to use "%PRIu64" instead of "%lu".

Fixes: 6336a810 ("KVM: selftests: replace assertion with warning in access_tracking_perf_test")
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221129175300.4052283-2-seanjc@google.com