ipv4: Unmask upper DSCP bits when building flow key

build_sk_flow_key() and __build_flow_key() are used to build an IPv4
flow key before calling one of the FIB lookup APIs.

Unmask the upper DSCP bits so that in the future the 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>