IB/ipath: Fix receiving UD messages with immediate data

This fixes a small bug in ipath_ud_rcv()'s handling of UD messages
with immediate data.  We need to test whether immediate data is
present and update the header size accordingly *before* testing the
packet size from the header against the actual received length.
Otherwise the wrong header size will be used and all messages with
immediate data will be dropped.

This bug keeps MVAPICH-UD and HP MPI from working at all on ipath devices.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>