UPSTREAM: zsmalloc: fix a null pointer dereference in destroy_handle_cache()

(cherry-pick from commit 02f7b4145da113683ad64c74bf64605e16b71789)

If zs_create_pool()->create_handle_cache()->kmem_cache_create() or
pool->name allocation fails, zs_create_pool()->destroy_handle_cache()
will dereference the NULL pool->handle_cachep.

Modify destroy_handle_cache() to avoid this.

Bug: 25951511

Change-Id: Ie4ea82fe34ac02e6e2548e6ed47257366d7b92f5
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>