s390/os_info: Initialize old os_info in standalone dump kernel

The commit be42660d0c13 ("s390/crash: use old os_info to create PT_LOAD headers")
introduced use of the old os_info into standalone dump kernel.
Before this change os_info_old_init() expected to be called only from
a regular kdump kernel although the function itself is able to work
in standalone dump kernels as well (because copy_oldmem_kernel() is able
to handle both use cases). Therefore, fix the expectation of os_info_old_init()
and enable it to be called from a standalone dump kernel.

Fixes: f4cac27d

 ("s390/crash: Use old os_info to create PT_LOAD headers")
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>