Skip the vm.pmap.kernel_maps sysctl by default.

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

Reviewed by: markj, kib
Differential Revision: https://reviews.freebsd.org/D27504

(cherry picked from commit 1dce7d9e7eefead038610df6a8d6c86a0fdbebb8)