kernel/sched: Fix uninitialized read in nohz_full/isolcpus setup

Fix reading uninitialized cpumask and using it to validate the nohz_full=
and isolcpus= kernel command line parameters.

An older version of a patch from lkml was incorporated into linux-yocto,
whereas a newer, rebased version was later published. See:
https://lore.kernel.org/lkml/20220221182009.1283-1-paul.gortmaker@windriver.com/



Signed-off-by: Adrian Cinal <adriancinal1@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>