RDMA/nes: Fix fast memory registration length

Zero high order word of fast memory registration (FMR) length field.
FMR length field is 32 bits, so high word should always be zero.

Signed-off-by: Tatyana Nikolova <Tatyana.E.Nikolova@intel.com>
Signed-off-by: Donald Wood <Donald.E.Wood@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>