sctp: fix invalid reference to the index variable of the iterator

Now in sctp_apply_peer_addr_params(), if SPP_IPV6_FLOWLABEL flag is set
and trans is NULL, it would use trans as the index variable to traverse
transport_addr_list, then trans is set as the last transport of it.

Later, if SPP_DSCP flag is set, it would enter into the wrong branch as
trans is actually an invalid reference.

So fix it by using a new index variable to traverse transport_addr_list
for both SPP_DSCP and SPP_IPV6_FLOWLABEL flags process.

Fixes: 0b0dce7a ("sctp: add spp_ipv6_flowlabel and spp_dscp for sctp_paddrparams")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>