s390/debug_config: enable kmemleak detector

...but set it to off by default. Use the kernel command line option
`kmemleak=on` to enable it.
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/configs/debug_defconfig

@@ -792,6 +792,8 @@ CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
 CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER=y
 CONFIG_SLUB_DEBUG_ON=y
 CONFIG_SLUB_STATS=y
+CONFIG_DEBUG_KMEMLEAK=y
+CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF=y
 CONFIG_DEBUG_STACK_USAGE=y
 CONFIG_DEBUG_VM=y
 CONFIG_DEBUG_VM_VMACACHE=y