[S390] fix alloc_pgste check in init_new_context

Processes started with kernel_execve from a kernel thread will have
current->mm==NULL. Reading current->mm->context.alloc_pgste will
read a more or less random bit from lowcore in this case. If the
bit turns out to be set the whole process tree started this way
will allocate page table extensions although they have no need
for it.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>