IB/qib: UD send with immediate receive completion has wrong size

The code to generate receive completion entries for UD send with
immediate contains the wrong payload length.  This is because when the
code to compute the payload size was moved, the value of hdrsize
didn't get moved too.  The fix is to update tlen directly.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>