IB/core: Check for rdma_protocol_ib only after validating port_num

port_num is untrusted data from the user, so it should be checked after
calling fill_sgid_attr, which validates it.

Fixes: 8d9ec9ad ("IB/core: Add a sgid_attr pointer to struct rdma_ah_attr")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>