IB/mlx4: Add VLAN support for IBoE

This patch allows IBoE traffic to be encapsulated in 802.1Q tagged
VLAN frames.  The VLAN tag is encoded in the GID and derived from it
by a simple computation.

The netdev notifier callback is modified to catch VLAN device
addition/removal and the port's GID table is updated to reflect the
change, so that for each netdevice there is an entry in the GID table.
When the port's GID table is exhausted, GID entries will not be added.
Only children of the main interfaces can add to the GID table; if a
VLAN interface is added on another VLAN interface (e.g. "vconfig add
eth2.6 8"), then that interfaces will not add an entry to the GID
table.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>