ipv4: Convert ip_route_input_rcu() to dscp_t.

ipv4: Convert ip_route_input_rcu() to dscp_t.

Pass a dscp_t variable to ip_route_input_rcu(), instead of a plain u8,
to prevent accidental setting of ECN bits in ->flowi4_tos.

Callers of ip_route_input_rcu() to consider are:

  * ip_route_input_noref(), which already has a dscp_t variable to pass
    as parameter. We just need to remove the inet_dscp_to_dsfield()
    conversion.

  * inet_rtm_getroute(), which receives a u8 from user space and needs
    to convert it with inet_dsfield_to_dscp().

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/c4dbb5aa9cbc79c4fcb317abbffa7c7156bc56a7.1727807926.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>