RDMA/rxe: Consider skb reserve space based on netdev of GID

Always consider the skb reserve space based on netdevice of the GID
attribute, regardless of vlan or non vlan netdevice.

Fixes: 43c9fc50

 ("rdma_rxe: make rxe work over 802.1q VLAN devices")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>