RDMA/hns: Remove redundant parameter "mailbox" in the mailbox

The parameter "out_param" of the mailbox is always null when the context is
destroyed. So remove the function parameter "mailbox".

Link: https://lore.kernel.org/r/20220302064830.61706-4-liangwenpeng@huawei.com


Signed-off-by: Wenpeng Liang <liangwenpeng@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>