RDMA/cma: Don't allow IPoIB port space for IBoE

This patch fixes a kernel crash in cma_set_qkey().

When the link layer is Ethernet, it is wrong to use IPoIB port space
since no IPoIB interface is available.  Specifically, setting the
Q_Key when port space is RDMA_PS_IPOIB requires MGID calculation and
an SA query, which doesn't make sense over Ethernet.

Signed-off-by: Moni Shoua <monis@mellanox.co.il>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>