ipv4: Convert ip_route_input_mc() to dscp_t.

ipv4: Convert ip_route_input_mc() to dscp_t.

Pass a dscp_t variable to ip_route_input_mc(), 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_mc(). 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: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/0cc653ef59bbc0a28881f706d34896c61eba9e01.1728302212.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>