s390/kdump: Account /sys/kernel/kexec_crash_size changes in OS info

The crashkernel size for kdump can be reduced at runtime with the
sysfs file "/sys/kernel/kexec_crash_size". Currently those changes
do not update the OS info crashkernel information that is used
for stand-alone kdump. With this fix now also the OS info crashkernel
information is updated correctly.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>