s390/kdump: Return NOTIFY_OK for all actions other than MEM_GOING_OFFLINE

We only have to check kdump memory for the MEM_GOING_OFFLINE action.
Therefore skip the test and return NOTIFY_OK for all other memory
hotplug actions.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>