ipv4: Unmask upper DSCP bits in RTM_GETROUTE output route lookup

Unmask the upper DSCP bits when looking up an output route via the
RTM_GETROUTE netlink message so that in the future the lookup could be
performed according to the full DSCP value.

No functional changes intended since the upper DSCP bits are masked when
comparing against the TOS selectors in FIB rules and routes.

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