sctp: process duplicated strreset in and addstrm in requests correctly

This patch is to fix the replay attack issue for strreset and addstrm in
requests.

When a duplicated strreset in or addstrm in request is received, reply it
with bad seqno if it's seqno < asoc->strreset_inseq - 2, and reply it with
the result saved in asoc if it's seqno >= asoc->strreset_inseq - 2.

For strreset in or addstrm in request, if the receiver side processes it
successfully, a strreset out or addstrm out request(as a response for that
request) will be sent back to peer. reconf_time will retransmit the out
request even if it's lost.

So when receiving a duplicated strreset in or addstrm in request and it's
result was performed, it shouldn't reply this request, but drop it instead.

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