IB/ehca: Construct MAD redirect replies from request MAD

The old code used a lot of hard-coded values, which might not be valid
in all environments (especially routed fabrics or partitioned
subnets).  Copy as much information as possible from the incoming
request to correct that.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>