ipv4: Convert ip_mc_validate_source() to dscp_t.

ipv4: Convert ip_mc_validate_source() to dscp_t.

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

Callers of ip_mc_validate_source() to consider are:

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

  * udp_v4_early_demux() which gets the DSCP directly from the IPv4
    header and can simply use the ip4h_dscp() helper.

Also, stop including net/inet_dscp.h in udp.c as we don't use any of
its declarations anymore.

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/c91b2cca04718b7ee6cf5b9c1d5b40507d65a8d4.1728302212.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>