netfilter: br_netfilter: Unmask upper DSCP bits in br_nf_pre_routing_finish()

Unmask upper DSCP bits when calling ip_route_output() so that in the
future it could perform the FIB lookup according to the full DSCP value.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>