netfilter: rpfilter: Convert rpfilter_mt() to dscp_t.

Use ip4h_dscp() instead of reading iph->tos directly.

ip4h_dscp() returns a dscp_t value which is temporarily converted back
to __u8 with inet_dscp_to_dsfield(). When converting ->flowi4_tos to
dscp_t in the future, we'll only have to remove that
inet_dscp_to_dsfield() call.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>