Currently if kmemleak is disabled, the kmemleak objects can never be freed,
no matter if it's disabled by a user or due to fatal errors.

Those objects can be a big waste of memory.

  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME
1200264 1197433  99%    0.30K  46164       26    369312K kmemleak_object

With this patch, internal objects will be freed immediately if kmemleak is
disabled explicitly by a user. If it's disabled due to a kmemleak error,
The user will be informed, and then he/she can reclaim memory with:

# echo off > /sys/kernel/debug/kmemleak

v2: use off handler instead of clear handler to do this, suggested
    by Catalin.

Signed-off-by: Li Zefan <lizefan@huawei.com>