RDMA/hns: Refactor the MTR creation flow

Split the hns_roce_mtr_create() into serval small functions, remove unused
member in 'struct hns_roce_buf_attr' and delete unnecessary MTR page count
check flow to make the MTR creation related codes clearer.

Link: https://lore.kernel.org/r/1611395282-991-2-git-send-email-liweihang@huawei.com
Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>