[IB] Fix RMPP receive length calculation
[IB] Fix RMPP receive length calculation

Based on simplification idea from Sean Hefty <sean.hefty@intel.com>

Signed-off-by: Hal Rosenstock <2518ffcbbe54672a7c659f2a6901d282d5f659bd@voltaire.com>
Signed-off-by: Roland Dreier <91e9b5f7ca0bb6300133ed378670d64af90dde66@cisco.com>

TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs