s390/smp: ensure global control register contents are in sync

Globally setting a bit in control registers is done with
smp_ctl_set_clear_bit(). This is using on_each_cpu() to execute a function
which actually sets the control register bit on each online CPU. This can
be problematic since on_each_cpu() does not prevent that new CPUs come
online while it is executed, which in turn means that control register
updates could be missing on new CPUs.

In order to prevent this problem make sure that global control register
contents cannot change until new CPUs have initialized their control
registers, and marked themselves online, so they are included in subsequent
on_each_cpu() calls.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>