IPoIB: Clear high octet in QP number

IPoIB assumes that high (reserved) octet in the hardware address is 0,
and copies it into the QPN.  This violates RFC 4391 (which requires
that the high 8 bits are ignored on receive), and will result in an
invalid QPN being used when interoperating with IPoIB connected mode.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>