sctp: use sizeof(__u16) for each stream number length instead of magic number

Now in stream reconf part there are still some places using magic
number 2 for each stream number length. To make it more readable,
this patch is to replace them with sizeof(__u16).

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