Encapsulate the code associated with the qp number assignment into
alloc_qpn() and free_qpn().

Link: https://lore.kernel.org/r/1582526258-13825-4-git-send-email-liweihang@huawei.comSigned-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>