s390/protvirt: parse prot_virt option in the decompressor

To make early kernel address space layout definition possible parse
prot_virt option in the decompressor and pass it to the uncompressed
kernel. This enables kasan to take ultravisor secure storage limit into
consideration and pre-define vmalloc position correctly.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>