ipv4: Convert ip_route_input_slow() to dscp_t.

ipv4: Convert ip_route_input_slow() to dscp_t.

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

Only ip_route_input_rcu() actually calls ip_route_input_slow(). Since
it already has a dscp_t variable to pass as parameter, we only need to
remove the inet_dscp_to_dsfield() conversion.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/d6bca5f87eea9e83a3861e6e05594cdd252583c9.1727807926.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>