RDMA/uverbs: Expand primary and alt AV port checks

The commit cited below checked that the port numbers provided in the
primary and alt AVs are legal.

That is sufficient to prevent a kernel panic. However, it is not
sufficient for correct operation.

In Linux, AVs (both primary and alt) must be completely self-described.
We do not accept an AV from userspace without an embedded port number.
(This has been the case since kernel 3.14 commit dbf727de
("IB/core: Use GID table in AH creation and dmac resolution")).

For the primary AV, this embedded port number must match the port number
specified with IB_QP_PORT.

We also expect the port number embedded in the alt AV to match the
alt_port_num value passed by the userspace driver in the modify_qp command
base structure.

Add these checks to modify_qp.

Cc: <stable@vger.kernel.org> # 4.16
Fixes: 5d4c05c3

 ("RDMA/uverbs: Sanitize user entered port numbers prior to access it")
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>