s390/dumpstack: fix address ranges for asynchronous and panic stack

git commit dc7ee00d "s390: lowcore stack pointer offsets"
introduced a regression in regard to show_stack(). The stack pointer
for the asynchronous and the panic stack in the lowcore now have an
additional offset applied to them. This offset needs to be taken into
account in the calculation for the low and high address for the stacks.

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