sctp: move the flush of ctrl chunks into its own function

Named sctp_outq_flush_ctrl and, with that, keep the contexts contained.

One small fix embedded is the reset of one_packet at every iteration.
This allows bundling of some control chunks in case they were preceeded by
another control chunk that cannot be bundled.

Other than this, it has the same behavior.

Changes since v2:
- Fixed panic reported by kbuild test robot if building with
  only up to this patch applied, due to bad parameter to
  sctp_outq_select_transport and by not initializing packet after
  calling sctp_outq_flush_ctrl.

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