s390/crash: Do not use VM info if os_info does not have it

The virtual memory information stored in os_info area is
required for creation of the kernel image PT_LOAD program
header for kernels since commit a2ec5bec56dd ("s390/mm:
uncouple physical vs virtual address spaces").

By contrast, if such information in os_info is absent the
PT_LOAD program header should not be created.

Currently the proper PT_LOAD program header is created for
kernels that contain the virtual memory information, but
for kernels without one an invalid header of zero size is
created. That in turn leads to stand-alone dump failures.

Use OS_INFO_KASLR_OFFSET variable to check whether os_info
is present or not (same as crash and makedumpfile tools do)
and based on that create or do not create the kernel image
PT_LOAD program header.

Fixes: f4cac27d

 ("s390/crash: Use old os_info to create PT_LOAD headers")
Tested-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Acked-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>