sysctl sysctls: wire old buf before output with sysctl lock

[rlibby]

Summary

Several sysctl sysctls output to a user buffer while holding a
non-sleepable lock that protects the sysctl topology. They need to wire
the output buffer, or else they may try to sleep on a page fault.

Sponsored by: Dell EMC Isilon

Test Plan

With D22527, check for witness warnings after
sysctl -ao > /dev/null