sctp: add sctp_packet_singleton

Factor out the code for generating singletons. It's used only once, but
helps to keep the context contained.

The const variables are to ease the reading of subsequent calls in there.

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