sctp: factor out stream->in allocation

There is 1 place allocating it and another reallocating. Move such
procedures to a common function.

v2: updated changelog

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