s390/smp: consolidate locking for smp_rescan()

Move locking to __smp_rescan() instead of duplicating it to all call sites.

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