sctp: move transport flush code out of sctp_outq_flush

To the new sctp_outq_flush_transports.

Comment on Nagle is outdated and removed. Nagle is performed earlier, while
checking if the chunk fits the packet: if the outq length is not enough to
fill the packet, it returns SCTP_XMIT_DELAY.

So by when it gets to sctp_outq_flush_transports, it has to go through all
enlisted transports.

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