The length information should be stored in the struct ib_mr object,
otherwise the length value of a valid mr object would always be 0.

Link: https://lore.kernel.org/r/1588931159-56875-7-git-send-email-liweihang@huawei.comSigned-off-by: Lang Cheng <chenglang@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>