net/vlan: Provide read access to the vlan egress map

Provide a method for read-only access to the vlan device egress mapping.

Do this by refactoring vlan_dev_get_egress_qos_mask() such that now it
receives as an argument the skb priority instead of pointer to the skb.

Such an access is needed for the IBoE stack where the control plane
goes through the network stack. This is an add-on step on top of commit
d4a96865 "net/route: export symbol ip_tos2prio" which allowed the RDMA-CM
to use ip_tos2prio.

Signed-off-by: Eyal Perry <eyalpe@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>