mm/zsmalloc.c: convert to use kmem_cache_zalloc in cache_alloc_zspage()

mm/zsmalloc.c: convert to use kmem_cache_zalloc in cache_alloc_zspage()

We always memset the zspage allocated via cache_alloc_zspage.  So it's
more convenient to use kmem_cache_zalloc in cache_alloc_zspage than caller
do it manually.

Link: https://lkml.kernel.org/r/20210114120032.25885-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-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>