RDMA/hns: Use refcount_t APIs for HEM

refcount_t is better than integer for reference counting, it will WARN on
overflow/underflow and avoid use-after-free risks.

Link: https://lore.kernel.org/r/1621589395-2435-5-git-send-email-liweihang@huawei.com


Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>