s390/boot: factor out offset_vmlinux_info() function

Move offsetting all of vmlinux_info fields to a separate
function for better readability.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>