RDMA/hns: Move the location for initializing tmp_len

When posted work request, it need to compute the length of
all sges of every wr and fill it into the msg_len field of
send wqe. Thus, While posting multiple wr,
tmp_len should be reinitialized to zero.

Fixes: 8b9b8d14 ("RDMA/hns: Fix the endian problem for hns")
Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>