sctp: add spp_ipv6_flowlabel and spp_dscp for sctp_paddrparams

sctp: add spp_ipv6_flowlabel and spp_dscp for sctp_paddrparams

spp_ipv6_flowlabel and spp_dscp are added in sctp_paddrparams in
this patch so that users could set sctp_sock/asoc/transport dscp
and flowlabel with spp_flags SPP_IPV6_FLOWLABEL or SPP_DSCP by
SCTP_PEER_ADDR_PARAMS , as described section 8.1.12 in RFC6458.

As said in last patch, it uses '| 0x100000' or '|0x1' to mark
flowlabel or dscp is set,  so that their values could be set
to 0.

Note that to guarantee that an old app built with old kernel
headers could work on the newer kernel, the param's check in
sctp_g/setsockopt_peer_addr_params() is also improved, which
follows the way that sctp_g/setsockopt_delayed_ack() or some
other sockopts' process that accept two types of params does.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>