RDMA/nes: Fix MSS calculation on RDMA path

Fix the calculation of the MSS for RDMA connections: we need to
allow space in frames for a VLAN tag too.

Signed-off-by: Chien Tung <ctung@neteffect.com>
Signed-off-by: Glenn Streiff <gstreiff@neteffect.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>