ipv6: sit: Unmask upper DSCP bits in ipip6_tunnel_xmit()

The function calls flowi4_init_output() to initialize an IPv4 flow key
with which it then performs a FIB lookup using ip_route_output_flow().

The 'tos' variable with which the TOS value in the IPv4 flow key
(flowi4_tos) is initialized contains the full DS field. Unmask the upper
DSCP bits so that in the future the FIB lookup could be performed
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>