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.comSigned-off-by: NChengchang Tang <tangchengchang@huawei.com>
Signed-off-by: NYixing Liu <liuyixing1@huawei.com>
Signed-off-by: NWenpeng Liang <liangwenpeng@huawei.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>