Skip the vm.pmap.kernel_maps sysctl by default.

[jhb]

Summary

This sysctl node generates very verbose output, so don't trigger it
for sysctl -a or sysctl vm.pmap.

Test Plan

• I tried to use 'sysctl vm.pmap' on my laptop to see the status of PTI and was flooded with many, many lines of output

[jhb]

Hmm, @freqlabs, I was testing this, and I found that while 'sysctl vm' skipped this sysctl, 'sysctl vm.pmap' did not, so it seems like something is broken with the CTLFLAG_SKIP handling.

[jhb]

To be clear, vm.pmap.kernel_maps is the node in question, so I would expect sysctl vm.pmap to skip it.

[freqlabs]

@jhb Thanks for the heads up. It's working as intended in other cases but clearly not in this case. I'm looking into it.