sctp: factor out stream->out allocation

There is 1 place allocating it and 2 other reallocating. Move such
procedures to a common function.

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>