ipv4: Convert ip_route_use_hint() to dscp_t.

ipv4: Convert ip_route_use_hint() to dscp_t.

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

Only ip_rcv_finish_core() actually calls ip_route_use_hint(). Use the
ip4h_dscp() helper to get the DSCP from the IPv4 header.

While there, modify the declaration of ip_route_use_hint() in
include/net/route.h so that it matches the prototype of its
implementation in net/ipv4/route.c.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/c40994fdf804db7a363d04fdee01bf48dddda676.1728302212.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>