sctp: add sctp_make_op_error_limited and reuse inner functions

The idea is quite similar to the old functions, but note that the _fixed
function wasn't "fixed" as in that it would generate a packet with a fixed
size, but rather limited/bounded to PMTU.

Also, now with sctp_mtu_payload(), we have a more accurate limit.

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