netfilter: flow_offload: Convert nft_flow_route() to dscp_t.

Use ip4h_dscp()instead of reading ip_hdr()->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.

Also, remove the comment about the net/ip.h include file, since it's
now required for the ip4h_dscp() helper too.

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