[S390] correct ipl parameter block safe guard

The 'output' variable is passed from decompress_kernel to
check_ipl_parmblock before it is initialized. That disables the
safe guard against the overwrite of the ipl parameter block.
Fix this by passing the correct value to check_ipl_parmblock.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>