s390/smp: kasan stack instrumentation support

smp_start_secondary function is called without DAT enabled. To avoid
disabling kasan instrumentation for entire arch/s390/kernel/smp.c
smp_start_secondary has been split in 2 parts. smp_start_secondary has
instrumentation disabled, it does minimal setup and enables DAT. Then
instrumentated __smp_start_secondary is called to do the rest.

__load_psw_mask function instrumentation has been disabled as well
to be able to call it from smp_start_secondary.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>