[VLAN]: Fix egress priority mappings leak.

These entries are allocated in vlan_dev_set_egress_priority,
but are never released and leaks on vlan device removal.

Drop these in vlan's ->uninit callback - after the device is
brought down and everyone is notified about it is going to
be unregistered.

Found during testing vlan netnsization patchset.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>