iw_cxgb4: Fix bug 2369 in OFED bugzilla

iw_cxgb4: Fix bug 2369 in OFED bugzilla

Bug 2369 - [OFED-3.5] Trace observed in dmesg while running IMB-MPI1 using
openmpi-1.6

dst_neigh_lookup and neigh_release function has been introduced
in kernel >= 3.4.0. neigh_release function was introduced to
decrement the reference acquired by dst_neigh_lookup function.
neigh_release was getting called unnecessary for the kernel < 3.4.0
which was resulting in the trace.

Signed-off-by: Vipul Pandya <vipul@chelsio.com>