RDMA/nes: Fix VLAN support

We need to account for the VLAN header size in nes_netdev_change_mtu()
and nes_netdev_init().  Also, add spin lock/unlock during VLAN RX
registration so only one process can assign VLAN group for a given
interface at a time.

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