[S390] Improve __smp_call_function_map.

There is no need to disable bottom halves when holding call_lock. Also
this could imply that it is legal to call smp_call_function* from
bh context, which it is not.
Also test if func will be executed locally before disabling
and aterwards enabling interrupts again. It's not necessary to disable
and enable interrupts each time __smp_call_function_map gets called.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>