sctp: update mid instead of ssn when doing stream and asoc reset

When using idata and doing stream and asoc reset, setting ssn with
0 could only clear the 1st 16 bits of mid.

So to make this work for both data and idata, it sets mid with 0
instead of ssn, and also mid_uo for unordered idata also need to
be cleared, as said in section 2.3.2 of RFC8260.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo R. Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>