sctp: add asoc and packet to sctp_flush_ctx

Pre-compute these so the compiler won't reload them (due to
no-strict-aliasing).

Changes since v2:
- Do not replace a return with a break in sctp_outq_flush_data

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