RDMA/hns: Use the core code to manage the fixed mmap entries

Add a new implementation for mmap by using the new mmap entry API. This
makes way for further use of the dynamic mmap allocator in this driver.

Link: https://lore.kernel.org/r/20211028105640.1056-1-liangwenpeng@huawei.com


Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Yixing Liu <liuyixing1@huawei.com>
Signed-off-by: Wenpeng Liang <liangwenpeng@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>