95znet: Make installation optional

znet_cio_free is a RedHat-specific tool, so do not install this
module on systems where the program is missing.

Signed-off-by: Hannes Reinecke <hare@suse.de>

modules.d/95znet/module-setup.sh

@@ -5,6 +5,7 @@
 # called by dracut
 check() {
     arch=$(uname -m)
+    [ -z /sbin/znet_cio_free ] || return 1
     [ "$arch" = "s390" -o "$arch" = "s390x" ] || return 1
 
     return 0