RDMA/hns: Add reference count for ucontext

driver inclusion
category: bugfix
bugzilla: NA
CVE: NA

Because OFED does not guarantee the order of execution of
hns_roce_vma_close and hns_roce_dealloc_ucontext, driver
needs to increase the reference count of the "context" to
ensure that the "context" will not be released when
hns_roce_dealloc_ucontext is executed first, until the vma
that depends on the "context" is completely released, then
the "context" is released.

Feature or Bugfix:Bugfix

Signed-off-by: Yangyang Li <liyangyang20@huawei.com>
Reviewed-by: wangxi <wangxi11@huawei.com>
Reviewed-by: oulijun <oulijun@huawei.com>
Reviewed-by: liuyixian <liuyixian@huawei.com>
Reviewed-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>