RDMA/hns: Bugfix for calculating qp buffer size

The buffer size of qp which used to allocate qp buffer space for storing
sqwqe and rqwqe will be the length of buffer space. The kernel driver will
use the buffer address and the same size to get the user memory. The same
size named buff_size of qp. According the algorithm of calculating, The
size of the two is not equal when users set the max sge of sq.

Fixes: b28ca7cc ("RDMA/hns: Limit extend sq sge num")
Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>