Prevent a superuser inside a jail from modifying the dedicated
root cpuset of that jail.
Processes inside the jail will still be able to change child sets.
A superuser outside of a jail will still be able to change the jail cpuset
and thus limit the number of cpus available to the jail.

Problem reported by: 000.fbsd@quip.cz (Miroslav Lachman)
PR: kern/134050
Reviewed by: jeff
MFC after: 3 weeks
X-MFC: backout r191596