sctp: reorder sctp_ulpevent and shrink msg_flags

The next patch needs 8 bytes in there. sctp_ulpevent has a hole due to
bad alignment; msg_flags is using 4 bytes while it actually uses only 2, so
we shrink it, and iif member (4 bytes) which can be easily fetched from
another place once the next patch is there, so we remove it and thus
creating space for 8 bytes.

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