s390/ctltreg: make initialization of control register save area explicit

s390/ctltreg: make initialization of control register save area explicit

Commit e1b9c2749af0 ("s390/smp: ensure global control register contents
are in sync") made the control register save area contained within the
lowcore at absolute address zero a resource which is used when
initializing CPUs. However this is anything but obvious from the code.

Add an ctlreg_init_save_area() function in order to make this explicit.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>