mm/mempolicy: convert from atomic_t to refcount_t on mempolicy->refcnt

mm/mempolicy: convert from atomic_t to refcount_t on mempolicy->refcnt

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.

Link: https://lkml.kernel.org/r/1626683671-64407-1-git-send-email-xiyuyang19@fudan.edu.cn
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Acked-by: Ben Widawsky <ben.widawsky@intel.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>