sysctl: Do not serialize requests when running as root

Bugs or unexpected behaviour can cause a user thread to block in a
sysctl handler for a long time. "procstat -kka" is the most useful tool
to see why this might happen, but it can block on sysctlmemlock too.

Since the purpose of this lock is merely to ensure userspace can't wire
too much memory, don't require it for requests from privileged threads.

PR: 282994
Reviewed by: kib, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47842