[S390] vdso: compile fix

!CONFIG_SMP:

arch/s390/kernel/vdso.c: In function 'vdso_init':
arch/s390/kernel/vdso.c:325: error: incompatible type for argument 2 of 'vdso_alloc_per_cpu'

Also move the code out of the BUG_ON statement since it won't be
executed on !CONFIG_BUG. And that would be a bug.

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