mm/demotion: fix NULL vs IS_ERR checking in memory_tier_init

alloc_memory_type() returns error pointers on error instead of NULL.  Use
IS_ERR() to check the return value to fix this.

Link: https://lkml.kernel.org/r/20221110030751.1627266-1-linmq006@gmail.com
Fixes: 7b88bda3761b ("mm/demotion/dax/kmem: set node's abstract distance to MEMTIER_DEFAULT_DAX_ADISTANCE")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Wei Xu <weixugc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
