s390/decompressor: specify __decompress() buf len to avoid overflow
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.4.232 by commit 16409f7d9ca5bb8220e1049ea9aae0d3c94d2dfb. For more details please see the references link.